Login Username is not working – site been hacked?

This afternoon I was in the backend of my wordpress site deleting some suspicious looking Registered users from the Plugin Subscribe2.
Suddenly I was redirected to my login page – I thought nothing of it at the time and logged in. It immediately said that there was no such username – I have tried to have a new password set and all sorts of different log in’s reopening browser windows etc….

I just not sure what to do next

Please help – I feel that I may have been hacked?