The User Profile Service Failed the Logon?

1. Boot into Safe Mode

Hold Shift + Restart > Troubleshoot > Advanced Options > Startup Settings > Restart > Press F4

2. Open Registry Editor

Press Win + R, type regedit

Go to:    

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList

3. Fix Profile Keys

Find folders starting with S-1-5-21

If one ends in .bak:

Rename the normal one (no .bak) to .bad

Rename the .bak one to remove .bak

4. Restart PC and try logging in

5.Still not working?

Create a new user account

Copy files from C:\Users\OldProfile to the new one

Delete the broken profile after confirming everything is moved