I upgraded my iPhone software yesterday and now I am getting personal hot spot. Can any body tell me why this and how to solve this problem.

Have your Carrier remove the feature, then re-add it. If that doesnt work, restore the software. 🙂

Did you check for a carrier settings update?

Copied from iOS: System requirements for Personal Hotspot

To confirm that your iPhone or iPad Wi-Fi + 4G can share its cellular data connection using Personal Hotspot, tapSettings > General > Network > Personal Hotspot.

If you don't see this option and your carrier supports this feature, check for a carrier settings update. If you still cannot enable this feature, contact your carrier for additional information.

Try the following steps:

Go to Settings>General>Network>Cellular Data Network and put your operator APN name in the APN field of the Internet Tethering. It should work.