Read axle is locked
 

Bought my son a used 110cc quad. Believe it's a Sunl. It starts no problem. It was running fine and now it won't move at all. Seems that the rear axle is locked and the quad won't move no matter how much gas you give it. Anyone have any ideas. Thanks.

Probably a cvt drive system, first thing I would do is to check to see if the drive belt broke and lodged itself in the clutch. Not an uncommon thing if the belt become excessively worn out.

I'd remove the sprocket and see if it's the rear end or in the motor/clutch.