Rear wheel on my tony hawk sypher huckjam bike is locked up

There are a few things that can be wrong, and based on what you said, you probably now have more than one fault. You need to isolate parts to find out what it is. Most of the of these checks can be done almost instantly without removing anything. The chain will get in the way - you won't need to remove it though.

1. Remove the wheel from the bike and check the freewheel. It should be moving freely in one direction and locked up in the other
2. Check the rear derailleur - is the chain running freely though it. If not, check the chain line and make sure the small sprockets are free spinning.
3. Check the chain line though the front end. Is the chain clear of the derailleur and sitting properly on the chain ring.
4. Spin the pedals and check the bottom bracket is not jammed up somehow. check the pedals and cranks are properly aligned and attached to the bottom bracket.