Lost Domain Admin password. Last resort?

So I got a call from an employee, his Brother in law has a business where he is running 1 server as a web server/FTP server for customers to send him files. He rarely has to log into it because his website has a management portal that he uses.

He had an “IT Consultant” set all this up in 2007. No backups, no updates, nothing. It just stays running doing its thing. Fast forward to this last week. He needs to update the SSL Certificate but he doesnt know the Domain Admin password, there are no other domain user accounts, and the Local server logon has been removed from the logon choices.

Am I missing any backdoors? or is a backup of the HDD and a reinstall/reconfig the last resort?

Thanks