You can try a device reset which may (or may not) help:
Device Reset (won't affect settings/data/music/apps/etc)
1. Press and hold (& continue to hold) BOTH the Sleep/Wake button & the Home button.
2. Continue to hold BOTH (ignoring any other messages that may show) until you see the Apple logo on the screen.
3. Release BOTH buttons when you see the Apple logo and allow the device to boot normally.
I'm on Verizon so I have no idea how Sprint enables/disables Personal Hotspot (it's account-based on Verizon). You may want to visit your local Sprint store and see if they can help.