I think you have a choice of two options. Either
1. Go to your onlide database - (probably accessible through your hosting control panel - phpMyAdmin), open your gallery database, select the 'wa_settings' table, and check your admin username and password. You can change them here if you like.
2. Delete the gallery folder from your remote server, and re-upload a fresh copy.
The first option would be a lot quicker, and that's what I'd try first.