Our 20 hp 46" cut Husqvarna '08 riding mower will crank just fine but will not move forward or backward.

Check to make sure the belt that runs from the engine pulley back to the transaxle is on all the pulleys.If it is a hydrostatic transaxle make sure the free wheel control lever at the back of the tractor is in correct position, or the brake plate may be stuck on but that would make a loud grinding sound.If it is a gear drive make sure the transaxle pulley is turning and make sure you didn't lose a rear wheel key from the axle,(if you had the tire off for a repair it is easy to lose the key when you go to put the tire back on.