I don't know if this is related to your issue but I had to set up ring tones for the two people I don't have set to default - and one ringtone I wanted to set had disappeared altogether. These are stock ringtones, not purchased downloads. Maybe yours has disappeared too. Don't have my phone with me to check as I'm at work right now.
I have noticed that the ringtones on ios8 are not working properly as sometimes on my iphone 6 the volume of the ringtones are very low and sometimes there very loud, has anyone had the same problems ?
the only way to get the volume back is to change the ringtone and then change it back again but after a couple of times it goes back to low again its a right pain.