I also found my hotspot stopped working when i upgraded to iphone 6. I called AT&T and we ended up with this situation that works for me (on both my windows8 and windows 7 laptop computers) :
1. make sure your security settings and key on the phone and the computer match each other ( on the computer, you can find them through the control panel).
2.In the settings on your phone, turn off hotspot.
3. turn off wifi on your phone.
4. Then turn on hotspot on the phone. When you get a message asking whether you want wifi turned on, say yes to that message (do NOT go back to the separate wifi settings for the phone).
5. look for your phone's name on your wifi connections on the computer - you may have to refresh several times to find the phone.
6. connect to the phone's wifi on the computer.
7. When you're done using the phone as a hotspot, go back to the phone settings and turn off hotspot. Then turn wifi back on in the phone's settings.