When you installed MediaWiki, you created a sysop user, default name WikiSysop (unless you changed it). It's a required part of the web-based installation.
Hit your wiki page Special:Listusers to see all the usernames on your wiki. The sysop user should be marked as a sysop.
If you don't remember the password, I imagine you could use mySQL directly to remove that user's password, then log in and change it. Or at worst, run the MediaWiki installation again (which should not affect your existing database).
DanB