My MacBook shut down unexpectedly. When I restart it it runs up to the point where I see log in screen. After I enter the password it shows my desktop and shuts down again. This happens repeatedly. What do I do?

Hey,

Try performing these basic troubleshooting steps:

Peripheral Problem

1. Shut down your Mac.
2. Disconnect all peripheral devices, such as hard drives or printers. If you have a desktop Mac, make sure that only a display, keyboard, and mouse or trackpad are connected.
3. Turn on your Mac.
4. Use your Mac for the amount of time that it would usually take for an unexpected restart to occur.
5. If an unexpected restart occurs, follow the steps to check for a Hardware error.
6. If an unexpected restart doesn't occur, turn off the Mac and connect one peripheral device at a time until an unexpected restart occurs.

Start in Safe Mode

Start up an Intel-based Mac in safe mode

1. On your Mac, choose Apple menu  > Shut Down.
2. After your Mac shuts down, wait 10 seconds.
3. Restart your Mac, then immediately press and hold the Shift key.
4. Release the Shift key when you see the login window.
5. Log in, wait for it to restart unexpectedly.
6. If it doesn't restart, then it's mostly a software issue.
7. Open System Preferences, go to Users & Groups
8. Choose your Account, then choose Login Items
9. Click the Lock in the bottom left of the System Preferences window and enter your Password.
10. Uncheck all the apps that are lined up for Startup
11. Press the lock again to confirm changes.
12. Open Finder, then go to Applications.
13. Move any apps that you don't want into the Bin by right-clicking the app and selecting Move to Bin
14. Restart normally and check if your Mac works.

Hardware Problem

Your Mac might have a Hardware issue. To check for Hardware issues, try this:

1. Turn on your Mac, then immediately press and hold the D key on your keyboard as your Mac starts up. 
2. Release when you see a progress bar or you're asked to choose a language.
3. Run Apple Diagnostics and post the results here.

I hope this helped. Please try all of these steps.

Reference links:

Start Apple Diagnostics: https://support.apple.com/en-us/HT202731

Startup in Safe Mode: https://support.apple.com/en-in/guide/mac-help/mh21245/mac

If your Mac restarted unexpectedly: https://support.apple.com/en-in/HT200553

Reply to this answer with the results or if you have questions.