Sheff Tom,
I too had the same problem but found a fix. 🙂
1. In Settings> *turn OFF* Wi-Fi, Blue-Tooth, Personal Hot Spot
2. In Settings>General>Cellular> *turn OFF* Personal Hot Spot.
3. Reboot Phone *hold on off switch AND home button at same time until phone turns off and apple reappears*
4. The go back into Settings>General>Cellular> *turn ON* Personal Hot Spot. In a few moments the iPhone Wi-Fi should show up in your computer and other devices as available.
4a. It phone doesnt show up; referesh your computeter (not iphone) Wi-Fi search.
...This is a known problem, especially with 4g users on various networks (especially verizon) as the 4g drops out when a phone call is recevied. If a phone call is picked up and lasts more than 90(+/-) seconds the Wi-Fi signal will be dropped. Repeat steps 1 through 4 above.