I'm afraid I'm running out of ideas . .  but here are a couple more things to check:

1. When logged in as host, look a the Admin-->Event Viewer to see if any related errors are being logged.
2. Have you checked the Recycle Bin to see if the extensions module might have been deleted from the Host-->Module Definitions page?

---

Bill, WESNet Designs
Team Lead - DotNetNuke Gallery Module Project (Not Actively Being Developed)
Extensions Forge Projects . . .
Current: UserExport, ContentDeJour, ePrayer, DNN NewsTicker, By Invitation
Coming Soon: FRBO-For Rent By Owner

Sure - use the Host --> Extensions page. In DNN 5, the Host --> Module Definitions page is simply the Extensions module filtered by extension type to show only modules and provide functions for Create Module, etc. Go to Host --> Extensions and click on the "Create New Extension" command in the action menu or (if shown by your current skin) the same action link at the bottom of the page. You can then set the drop-down for "Select Extension Type" to "Module" and you'll have the same capabilities for registering your module definition and module controls as found on the Module Definitions page.

---

Bill, WESNet Designs
Team Lead - DotNetNuke Gallery Module Project (Not Actively Being Developed)
Extensions Forge Projects . . .
Current: UserExport, ContentDeJour, ePrayer, DNN NewsTicker, By Invitation
Coming Soon: FRBO-For Rent By Owner