I've had a snitz forum for years and have always had the same admin password, suddenly today it said my username or password was wrong, tried it several times on two different computers and it wouldn't work. In the end I used the Lost Password function which emailed me the link to reset the password and setup a new one and that worked fine.
Now could the system just have lost my password for some strange reason or has someone managed to find out my password, log in as me and change it?
I'd guess the latter - immediately look through your members in case you see any added accounts with admin privs - that's usually the first thing they'll do, make a new (often system-looking) named admin account and use that iun the future.
If you can take the forum offline for a half-day, download the database and search for anyoen with an m_lev of 3