This happened to me too.
I found the following solution somewhere on this forum and can't find it again...
1. Go to:
Settings > General > Cellular > Cellular Data and press the Reset Settings button.
2. Immediately turn iPhone Off using power button at the top.
3. When you start it back up the Personal Hotspot option magically re-appears in
Settings > General > Cellular
Perhaps it will be fixed in 6.1.4?