New user overwrote Admin user

I have a blog/site set-up and am having a huge issue.

The details of the set-up:
wordpress 3.01
Wishlist Member plugin activated to manage membership site
Multi-site activated – but I don’t believe this person doesn’t have a site set-up yet…

Someone registered at 9:33 pm this evening and their membership information overwrote the “admin” user. I’m not sure how to get the admin user back … how to re-add the admin user and how to make sure that the admin isn’t overwritten again. I’m not sure why this happened, could there be a limit in the SQL database?

Thank you in advance for the time and assistance.

Liz Martinez