If you didn't loosen the track before you stored your sled, you now risk having the track bonded to your rear idler wheels. Rotate the track by hand, slowly, to see if you can dislodge them. Worst case scenario here is that you may need some new rear idlers. Inspect your track. Make sure that there are no holes or rips in it.
Some companies advertise that they can repair tracks.
Check with them to see if your's qualifies. Check the track for proper tension and alignment. Each model varies so check with your manual. Start with the front and move back. Check to see if your shocks are leaking.