Try purging the transmission, see "purging transmission" in the owners manual.
Make sure you haven't mistaken the mower belt for the drive belt then the problem could be the drive belt is broken or something. The easiest way to identify the drive belt is it's the belt that goes to the transaxle pulley.
Note my 25 HP 48" modle 917.273241 tractor had a extreme drive belt slippage problem due to a too small of a spring on the idler pulley. If yours has the same problem (transmission pulley isn't turning when the clutch and drive petal are engaged) you can verify that with someone checking that using a mirror while the other person is operating the tractor.
This message was modified Apr 14, 2007 by A9000