Windows 10 login screen blocks the admin account (only account) -

We have a new dell business computer (don't have model with me now, but its an optiplex with serial and ps2 ports in the back, purchased a few months ago with i5 processor + 8 gigs ram) and our hard drive went bad and dell replaced it. Then a little later we had an issue with our windows genuine activation saying its not genuine when we had windows 7 and it blocked the admin account on the sign in screen (only account). Meaning you can't do anything, there are no other options, no option to enter key code or make genuine or anything like that. 

So I reinstalled Windows 7 (Dell CD) + downloaded drivers (except sata drivers which it didn't show it needed in device manager and I was pushed to finish quickly) from dell  today, thought all was well. It showed genuine and activated, etc... 

Employee upgraded to windows 10. Guess what? The login screen when you turn on the computer has once again blocked the admin account which is the only account. What is going on and how do I fix it? This is ridiculous. I can't even login to try to fix it.

edit: forgot to mention that this computer was added to a domain at work, not sure if that makes a difference or not