IPhone – Macbook to iPhone tether, hotspot or wifi

There is no built-in logic to specify an if/then decision to not join WiFi so the mac will join any WiFi SSSID that is detected and matches its list of auto-join networks.

You can set the service order by "secondary clicking" the gear at the bottom of the Network Preference Panes to Set Service Order... and make sure the WiFi is lower than the USB connection. It will still join, but no traffic will flow while the USB connection is viable.

Lastly, you could just make a new Network location called USB Only and turn off WiFi all together when you select that location on the mac. Changing network locations can be scripted as a service and AppleScript/Automater or FastScripts to effect or reverse the change you desire from a keyboard shortcut.

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.