Did you already try to set it up again "as new device" without using the last backup? How to back up your data and set up as a new device
If this does not bring back your ringtone, you might have to get it serviced.
1. Do the basic troubleshooting steps described in the User Guide, Appendix B:
Restore in iTunes using a backup
Restore as new, without using a backup/
If restoring as new doesn't help, there is a hardware failure that requires evaluation by Apple or whoever provides iPhone warranty service in your county.