Do you have phpmyadmin installed so that I can take a look at your tables? If so, PM me the link. Otherwise you'll need to look at the usergroups table and possibly add them manually:
INSERT INTO usergroups VALUES ('1','Anonymous','','','','','','',0,0,0,0,0,0);
INSERT INTO usergroups VALUES ('2','Admin',1,1,1,'','','',1,1,1,1,0,1);
INSERT INTO usergroups VALUES ('3','Regular Users','',1,1,'','','',1,1,0,1,0,0);
It's likely that whatever issue was causing this was resolved with the recentl 5.1x updates; you should upgrade to 5.12 as soon as possible as that was a security update.
Please do not PM me for support or sales questions. Thank you for your understanding.