I had an issue recently where a computer joined the domain while sysprep was still active and a group policy with password complexity broke the admin account that sysprep was using. To fix this we had to run a command prompt with admin permissions to run some commands, but we weren't able to start an elevated prompt.
The fix is to run the following command
runas /netonly /user:computername\adminaccount cmd
This will open an elevated command prompt. You can substitute cmd with anything else to run it as an admin.