Wierd stuff happened.
 

Okay, out of nowhere, it starts ipbanning random players/staff members, I was one of the users that got banned. What happened was when you logout it automatically bans you then staff members could just go to /admincp/ and login their, and they were unbanned, regular members had to register a new account, then login. What has happened to my vBulletin forums?

Please help.

This doesn't happen with a default vbulletin board - they don't suddenly get gremlins. Most likely it is a modification causing the problem. Try disabling your modifications and see if this still happens.
Note: To temporarily disable the plugin system, edit config.php and add this line right under <?php

It started happening out of the blue, where as we havn't installed a new modification in months, and it randomly started happening.

Have you looked at your access_logs (if you don'tknow where they are, ask your host) to see if someone is doing this to you? Or, have you looked in your Control Panel logs to see if a mod is doing it? Or see if a script is doing it? Have you looked on your server to see if there are any files on there that shouldn't be there?

Yes, all of the above.

Either someone has installed or changed something, or someone has access to your server to do this. Things cannot just randomly happen like that.