why do my downloaded ringtones for my galaxy prevail keep resetting?

[lilmiss0303]

I just got the phone last night and i downloaded some ringtones. I assigned the one i wanted for incoming calls and when i got up this morning it had reset back to the original ringtone....how do i save my selections and stop it from resetting?
Thanks but the ringtones arent deleted that i downloaded i have no problem getting the ringtones i want they just wont stay assigned to my incoming calls its driving me crazy why does it keep resetting the next day back to the original ringtone the phone started with???? HELP