Possible reason it self deleted is because possibly you connected your phone to your computer via usb, mounted the sd cards, then you didn't do a safe remove. Meaning you might have just yanked the usb out instead of asking your computer nicely to pull out usb safely. This can delete and corrupt files on sd cards.
I've also been reading on various forums, that if you pull the sd card out, or tinker with it, the movie self deletes. Some people are reporting this, however, I did pull my sd card out, put it into a different phone, and put it back into my vibrant, and the movie is still there.
3rd possibility, your sd card is unmounted...go to settings...sd cards and phone storage...and check to see if the sd card is mounted to your phone.
However, just to be safe, i've already backed up the movie onto my desktop.
If you do a factory restore, this should restore the avatar icon possibly, and then you just need to get the avatar movie file and load it back onto your sd card.
should be a simple fix.