Have you cleaned it recently?
If crud has gotten into the slot through which the hand (that is, the lever which advances the cylinder) operates, the hand won't contact the cylinder's centrally-located ratchet. Thus, the cylinder won't rotate.
If that's not it, then either the hand itself is damaged in some way, or the ratchet on the cylinder is.

If you can't fix this problem with a gun-scrubber spray and/or a toothpick, you need a gunsmith.
Do not try to take this pistol apart on your own!