Check the Sprint down detector and see if you are in an impacted area.
You might also check with customer service and see if there are any reports of tower issues in your area and ask that your account be reprovisioned as you are experiencing service issues. That will at least eliminate account/network issues.
If your phone is unlocked, see if you can borrow a sim from a different carrier for test purposes.
On the phone itself, consider following the steps for a frozen phone, and if that does not help, a restore via iTunes. After that, you should contact Apple Support or perform an Update to install a new copy of the operating system.