Occasionally, when restarting your Mac, the computer may stall and not progress any further with the startup process. Depending on your Mac model, the screen may be gray, black, or very dark. You might see the Apple logo, a spinning gear, a spinning globe, a circle with a slash, or nothing but a blank screen. There are troubleshooting steps and solutions to try to get your Mac back up and running.

The stalled startup is referred to as the "gray screen" problem, even if the display shows a black screen that looks like a powered-off Mac. Earlier Mac models showed a gray screen during a stalled startup, and the name stuck.

How to Fix the Gray Startup Screen on Mac

Before you try any of these troubleshooting steps, power down your Mac, then remove the Ethernet cable, audio in and out cables, headphones, and other peripherals. Keep your keyboard, mouse, and display connected.

1. Power on your Mac without peripherals attached. If your Mac starts up without issue when you don't have any peripherals attached, you'll know that a faulty peripheral or cable caused the gray screen problem. Shut down your Mac again, reconnect a peripheral, and then restart your Mac. Continue this process until you find the bad peripheral.

   The problem might also be a bad cable. If you plug in a peripheral and it causes the gray screen issue, try a new cable before replacing the peripheral.

2. Swap out your mouse and keyboard. If you've gone through your peripherals and none of them seem to be the culprit, the mouse or keyboard may be the problem. Connect a known, good mouse and keyboard, then restart your Mac. See if this solves the gray screen problem.

3. Start your Mac using the Safe Boot process. Make sure all peripherals, except the mouse and keyboard, are disconnected, then try a Safe Boot. During a Safe Boot, your Mac performs a startup drive directory check. If the drive directory is intact, macOS continues the startup process by loading only the minimum number of kernel extensions it needs to boot.

   If your Mac successfully starts up in Safe Boot mode, restart your Mac again in normal mode and see if it's able to complete the restart process.

   If your Mac is able to start after a Safe Boot, it may need some drive repairs. Use Disk Utility's First Aid tools to check and repair your drive.

4. Reset PRAM and reset SMC. If you can't start your Mac in Safe Boot mode, reset the PRAM and SMC, then see if your Mac will boot up. If it does, reattach your peripherals one at a time, restarting the Mac each time, to verify that none of them caused the original gray screen issue.

   Resetting the PRAM and the SMC returns your Mac's hardware to its default settings. For instance, sound levels are set to the default, the internal speakers are set as the source of audio output, and the display options and brightness are reset.

5. Remove extra RAM. If you added any RAM to your Mac after you bought it, remove that RAM, and then see if your Mac starts normally. If it does, then one or more RAM pieces have failed, and you'll need to replace it. You should be able to continue working with your Mac until you get the replacement RAM.

   We're getting to the point where the troubleshooting steps will likely cause you to lose some, if not all, the data on your startup drive.

6. Boot from a bootable installer. If you successfully boot from an external media with a bootable OS, such as a USB flash drive, you'll know the startup drive is causing your gray screen problem. (Alternatively, start your Mac from the Recovery HD.)

   If you identified the startup drive as the problem source, repair it using Disk Utility's First Aid.

   If you can't repair your startup drive using Disk Utility, try a third-party drive utility. If the repair process isn't working, you may need to replace your startup drive.

7. Boot into single-user mode (fsck). The little-known single-use startup mode boots the Mac to a screen that displays information about the startup process. (It looks like an old-fashioned terminal.) It notifies you if problems were found with your startup disk and attempts to correct the errors. If it successfully repairs the startup drive, you'll see a "Volume OK" message. If you don't see this message, the startup disk has bigger problems.

8. Make an Apple Store appointment or find an Apple-authorized service center. If none of these troubleshooting steps have been successful, it may be time to replace your startup drive or explore problems with another internal component. Be sure to take your Mac to a trusted entity.

Causes of the Gray Screen Startup Problem

There are several types of Mac startup problems, but the gray screen issue is particularly frustrating because there are many possible causes. Problems with peripherals, RAM issues, or drive problems could be preventing your Mac from starting up properly.

For example, when a bad peripheral is plugged into your Mac, it can prevent the Mac from continuing the startup sequence, causing the Mac to stall while it waits for the peripheral to respond to a command. Sometimes, a bad peripheral or its cable causes one of the signaling pins on one of Mac's ports to get stuck in one condition (set high, set low, or shorted out to ground or positive voltage). Any of these conditions can cause your Mac to freeze during the startup process.

Whatever the cause of the gray screen problem, there are several troubleshooting steps to try to get your Mac back to work.

The gray screen problem presents in different ways. In Mac models with built-in Retina displays, the computer may freeze on a black screen, making the Mac appear powered off. In other models, you may see a blue startup screen that changes to a stuck gray screen. You may see an Apple logo or an icon, or just a gray or dark screen.

• What do I do if my MacBook screen starts displaying grey lines?

  If your MacBook is still functional and the screen is working, but you keep seeing grey lines, it's probably due to the hardware overheating. Try performing a Safe Boot first, but resetting NVRAM, PRAM, or SMC might also fix the problem. Another potential workaround is to reduce the amount of strain on your MacBook by letting it rest after eight or more hours of use, and turning off screen savers.

• Why is my MacBook showing a grey screen when I wake it up from sleep?

  The problem is likely related to a login or startup item, or otherwise an issue with your user account. Restart your MacBook in Safe Mode, and if the problem goes away run Disk Utility to repair your startup disk. If the problem persists in Safe Mode it might be your user account, in which case you'll want to log out and log back in as a different user to see if that fixes things. If the problem goes away under a different account, you may need to replace your user account.