Trouble logging into Admin

My blog was hacked a few weeks ago, and it disappeared. My host recovered my files and everything is fine except now I can’t log in to the admin area. The login screen is missing everything except for the user and password fields, and when trying to log in, I am transferred to a page that says:
Error 403 – Forbidden
You tried to access a document for which you don’t have privileges.

That’s what was showing when the blog was hacked into. Somebody registered with the blog and they were able to change my files somehow. I still don’t know how they were able to do this.
I need your help.