According to readers there is this issue of personal hotspot missing after iOS update which means that as soon as they update to iOS 5.1 their Personal Hotspot goes missing on their iPhone. The very same issue had arisen with the iOS 5 updating too. It may or may not be a bug but the good thing is that the fix for this has been worked out and is quite an easy one too.
Fix Personal Hotspot Missing in iPhone
Go in Settings and click on “General” and scroll down at the bottom and tap on the “Reset”
Now click on “Reset Network Settings”
Then go back to Settings and there you will find the Personal Hotspot option
You need to save the important network settings before resetting the network preferences or else you will lose your wireless network passwords as well as custom DNS settings.
A second way to fix the Personal Hotspot missing is to retype the access point name through Internet Tethering settings:
You will find APN under “Internet Tethering”
Type in an access point name. You won’t require username and password.
Jump to our How-to section for more tips and tricks tutorials.