iPhone 4S crashed, Wifi crashed, Personal Hotspot gone
Since there wasn't physical trauma, you should try to restore its factory settings through iTunes.

You can try to restore your backup afterwards (iTunes will ask you if you want to or use the phone as a new iPhone), and see if it's still working. That should bring back your Personal Hotspot.

If not, re-restore it and set it up as a new iPhone. You'll lost your messages and some settings. But you should be able to keep your contacts (if sync through iCloud, Gmail, or other), and your mails.

If it's still not working, bring it back to an Apple Store, they'll most likely give you a new one.

Since the Personal Hotspot feature is controlled by your carrier, so you might also want to check with them if it's still activated on your account.