screwed user registration

hello all!

a few days ago I tried to install Hyper Cache. Since I am a beginner, I messed up things quite ugly and got to deleting complete site then restoring from backup (including mysql databases).
The problem is that now new users cannot register and I have no idea where to start troubleshooting.
I wanted to give a search on existing posts here, but the engine gave me thousands of results and i do not have the time to find my exact case. Please apologize for this.
Can anyone help?