Mail app "locked" refuses to open, error message

[Reibs]

Has anyone seen this error:

The mailbox ~/Library/Mail/Mailboxes/INBOX.mbox is currently locked by (null) on host . If you open a mailbox that already is in use, you may damage its contents.

Heres what happened: - attempting to get e-mail, the mac freezes up when mail app is starting, i forced quit the mail program and restarted, however, when I re-booted, it would only start in OS 9, tried 4 times to re-boot, each time only OS9 would start. I had to use Disk Utility -> First Aid to repair the "macintosh hd disk". That worked fine, then when I re-booted, it started up with OSX, however Mail gives me the above mentioned error... any suggestions?

Any help would be appreciated, thanks