Check the control rods that work the transmission/differential. they might have come off the levers that they hook onto, that change the transmission/differential movement. Hard to say, but if you didn't cover it up before winter came--shame on you! Lawn tractors are not like little cars. they have parts under them that tend to get filled with dirt, grass clippings, small sticks, etc. And, if it sits outside, not covered up, and the weather turns really cold, all of the old dead grass and dirt collection freezes, and won't allow the shifter to operate like it does in warmer weather.
Buy some PB Blaster penetrating liquid, and thoroughly bathe all of the moving parts in the vicinity of the top of the transmission. Then check the brake mechanism, usually a little disc and brake caliper, on the right front side of the trans., and bathe it in the penetrating oil, PB Blaster. It won't hurt anything to do that, and might solve your problem.
Trouble is---folks tend to think those little machines are like little cars, or trucks, and they can set outside, or under a lean-to, in the snow, cold, wind, and rain! They shouldn't have that kind of treatment. And, a bit of pre-winter treatment helps immensely!

IF you go to
searspartsdirect.com
and enter your tractor model#, you can get "their" drawing of the transmission & parts list.

It might be helpful to see what you're up against.
Seems like possibly a shifter fork issue, but.....

Cleaned thouroughly, lubed, oil changed, filters changed and cleaned before storing in the garage for the winter.

Though your description sounds as though it is internal in the transmission, there are a few things to check. You need to KNOW that the belt is turning the transmission pulley. You also need to know that the brake is free and the tractor is able to roll in nuetral as Rusty said. If all OK this far, remove the hub caps and as look to see as you are "driving" that an axle shaft isn't turning within either wheel. Those things done and still no drive, it's time for transmission repair. Let us know what you find.

OP - ck yer park brake mechanism at tranny to make sure it's not froze up or sticking.

OHHH?? Didn't i mention that? Sorry! RJ

My bad, Rusty - missed yer earlier reply to OP

Thats ok--been there/ done that, too.
Any way, only a few more months of looking at the snow! RJ

If moisture collects on top of the trans, it sits in the
recess in the middle and seeps into the trans via the center fastener hole. It usually rusts the shifter/detent
area and causes the shifter to freeze.
That likely was your original problem, but there may be
other damage done while trying to free things up.
On the top of the case, there should be two small allen
head/torx setscrews there, you may have to remove the pulley to get to them, the battery as well, if it is located under the seat.
Remove the two setscrews, underneath, there should be a spring, and under that, a steel ball, and rust.
Remove the set screw, and spray lube, wd-40, pb blaster,
etc., in the hole and let it sit, and gently try to
move the shifter.Repeat until free.
If there is other damge, then disassembly will be needed, and in the long run, you should disassemble and
clean out the rust anyway, but this should get your shifter moving for now.

Fish

Great ideas, thanks so much. Saved me a couple of hundred bucks. Pully on top of the trany turning, brake disengaged, BUT the right axel shaft was turning inside the wheel. Somehow or another, the key that locks the wheel to the axel got pushed out, not sure how to explain how THAT happened but got all put back together and problem solved.

I own a Craftsman Lawn Tractor 21.5 HP, 42 " 6 speed transy model # 917.272450. I have a copy of the Lubrication chart and can find all four of the greese fittings in the front but am unable to find the geer-shift pivots in the back. Can someone please instruct me as to how to find these Pivots so I can grease them? Thanks. My email address is gwd3d@yahoo.com.

If you are expecting to find grease fittings (zerks) on the gear shift pivots.........you ain't gonna find any.
The lube chart you are viewing has footnote numbers next to lube items that tell what kind of lubricant is to be used.
Zerks are footnote 2, which is General Purpose Grease.
The shifter linkage pivots are footnote 1, which is simply Motor oil.
Now, if you mean you can't FIND the pivots, just look under the fender and follow the linkage that connects to the shift lever......and oil every piece of linkage that goes through a bushing.
I hope this helps.