Disable the 'Authentication - JFBConnect Social Integration', 'System - JFBCSystem', and 'User - JFBConnect Social Integration' plugins. This will ensure that JFBConnect is not active. Then try to log in with your Joomla login credentials through the standard Joomla Login module. If you're still having this issue, this would point to some other underlying cause on your site. If your database tables are crashing repeatedly, it's also an indication that something is probably wrong on the server.