Can't Log In - redirecting to Login Page

Hi, I have just installed this plugin. It suggested I change the database prefix so I did to mit, then it suggested I changed the admin username - I didn't think there was an admin username as I use my name, alice, as the superadmin username. It straight away logged me out, and now when I try to login it is redirecting me back to the login screen. Sorry I can't grant you access as I am logged out! Please help!
Alice.