If the account was changed from Administrator to a standard user and there is no other administrator account available, use Safe Mode to restore admin access:
- Hold Shift while selecting Restart in Windows.
- Go to Troubleshoot > Advanced Options > Startup Settings.
- Select Restart.
- Choose option 4 to start in Safe Mode.
- When prompted for a password, leave it blank and continue.
- In Safe Mode, press Windows + R, type netplwiz, and press Enter.
- Double-click the affected account.
- Open the Group Membership tab.
- Change it back to Administrator.
- Restart the PC and sign in normally.
If an Administrator account is visible but disabled, it can also be re-enabled:
- In the search box, type computer management and open Computer Management.
- Select Local Users and Groups > Users.
- If Administrator shows a down arrow, double-click it.
- Clear Account is disabled and select Apply.
- Sign out and sign back in with the Administrator account.
If the sign-in screen is only showing the built-in Administrator account after a reset, selecting the password box for that account can make the other user accounts appear in the lower-left corner.
References:
- Find lost files after upgrading Windows
- administrator - Microsoft Q&A
- disabled administration rights but now unable to restore it - Microsoft Q&A
- I accidently changed the admin account to an owner account and I can't get into it anymore. - Microsoft Q&A
- Windows 10 may display the Administrator account at the sign-in screen after resetting the PC