If somebody has stolen your admin account and changed the password so that you can no longer login, all is not lost provided you can still get direct access to the database.
The solution will involve working directly on your database. Specifically your Drupal database will have a users table. For securty reasons you will not be able to see any passwords in this table because they are all hashed. However, if you create a new user with a known password then you can apply that password to your old admin account in order to recover it,
The last thing to consider is how you can prevent this from happening again. Do you have a security hole in either your online or offline operations that needs to be fixed?