Why did my phone lose the ringtone configuration?
I don't know if it could be your case, but if your phone is connected to your computer with UMS (USB Mass Storage) enabled along having a custom ringtone stored on your SD card (/ringtones folder) instead of MTP this will give you this kind of problem.

The phone will use a default ringtone since it cannot access the SD card files, because the UMS mode requires exclusive access to the mass storage.