Help! I think our site has been hacked.

Hi,

I received an email to say a new user has been added to my word press account. This shouldn’t happen as nobody knows the password. Anyway, I think they have changed the admin rights for my account because now I cant even find the list of users to remove this new person. It seems like they have reduced my admin rights or something? The site still functions like normal but I am worried that they could delete users or mess with the site. Has anyone experienced this before and if so what is the best solution? Your assistance would be greatly appreciated!

Luke