Can you log in in multi-user mode?

If so, you can find out who is logged in by selecting either File, Properties or if you are using the current version, File, View All Users.

If you can't log in in multi-user mode, then it will likely give you the message that you are already logged in and your username is the problem.

Then log in in multi-user mode as sysadmin, then close the file completely and let a minute go by.

Then open the file and log back in again as sysadmin in single-user mode.  If it allows you in, you can close the file and open it again as your original username in single-user mode.