Where did you store them, and what have you done lately with your device. Things don't just vanish, without either deleting, or possibly wiping when you upgrade. Only other explanation would be those files on your SD were corrupted, and that's why you can't see them, but that should effect your device, by freezing it when the card is in.
I saved them to "My Ringtones" on the internal memory. They worked fine for three months then BAM, gone.
Six more that I downloaded at the same time are still there.
Most likely lost due to low memory. Best to get an SD card, even a cheap one, and keep all your ringtones and pictures on it. Here's a good, inexpensive source:
I hear tell you�re doin� well,
Good things have come to you.
I wish I had your good-luck charm,
And you had a do-wacka-do, wacka do, wacka-do, wacka-do.
Posted from my CrackBerry at wapforums.crackberry.com