Pull the front wheels off the mower and see if the plastic gears inside the wheel hub are stripped. Also check and be sure the drive belt is still on both pulleys.
If you'll share a serial number with me, I can find out if you have a factory warranty left; the repairs needed may be covered. The serial number is on a sticker on the back of the mower deck and starts in the format "MXXX-1234567"
If you know you are out of warranty, then, wow/when did the problem start? All of a sudden, or did it start slower, slower, slower and then no drive at all?
Belt may have come off, that would be first this to look at...