Password loop

Hello,
Thank you for your time. Let me explain the situation.

We had an employee of a friend help set up wordpress for us. She set wordpress up on three different sights for us. She no longer works for my friends company and the e-mail address she had registered under this word press account is no longer active.

I have been managing and upgrading the websites without trouble as I have the login information. However one of these sites has stopped working properly. The site itself still functions, I am having difficulty logging in. It is stuck in a password loop. I enter an incorrect password it shows the usual error message but when I enter the correct password it returns to the default login screen as if I had not even done a thing.

We need to update this website. What can I do.
My employer is breathing down my neck on this one. Help would be greatly appreciated.

Cheers,
Craig