If your Personal Hotspot disappears on iOS 6, you have two options before reinstalling and / or restoring from backup:

A. go to Settings > General > Cellular and toggle ‘Cellular Data’ and ‘Enable 3G’ off and back on – then check of the ‘Personal Hotspot’ has come back;

B. if A above fails, go to Settings > General > Reset and ‘Reset Network Settings’, note this will also clear any custom network related settings – so be warned.

If all this fails and even a restore does not work, please check with your mobile carrier that you are on a data plan.