You can unban yourself.
Use this to delete banned_user.cfg:
http://aluigi.altervista.org/adv/sourceupfile-adv.txtand this to make the server crash/reboot (if it has auto reboot), so you are no longer banned:
http://aluigi.altervista.org/adv/sourcefs-adv.txtOr you can just wait until the next server reboot if you don't want to crash it.
If you weren't banned by banned_user.cfg but with source bans for example, you can delete sourcemod.vdf, so sourcemod (sourcebans) no longer loads and you are unbanned :)
But I'm not sure the admin will appreciate that you delete the files and crash his server... :X
I don't think there is an other way to get unbanned.