This sometimes happens if you leave the VBE open and the computer goes to sleep or hibernates. It's difficult to reproduce, but there's a simple fix -
Just close the VBE window and reopen it. I know, this is your default first troubleshooting step, but you forgot it in haste. It's OK, I won't tell anyone.
MS KB829059 talks about Outlook 2003 and 2007, but this applies across the entire Office Suite in 2003 and 2007.