Problem: Can’t use personal hotspots
After upgrading to iOS 7.1, some people complained that they can no longer use personal hotspots.
If your phone has this issue, we’re afraid to say that you’re out of luck. Currently, there is no fix available. Apple’s support staff and carriers around the world seem to have a problem pinning down the source of the malfunction, and have instead resorted to passing the blame on each other.
Try to update your ios with 8.0.1 or higher version. May be its your luck if its works.