If it were blocking you because of number of IPs, you'd see an error. That said, it shouldn't apply any restriction to an admin account. Could you look in your server for an errors log and see if something at the bottom is related to this problem?
You can remove the s2Member directores from the plugins using your FTP client. It won't remove the info in the database, but that shouldn't keep you from logging in. There may be a conflict with another plugin, not sure if you updated anything right before the problem.
Also, this thread may help in case you want to try re-installing.
viewtopic.php?f=36&t=247I hope this helps.