Well, it's going to spin in circles(that's how gas and nitro engines work), but it it's not making any sound, that's not good. If you turn the shaft by hand, does it offer much resistance? If it spins very freely(as if the shaft was just spinning on bearings), then you may have broken the connecting rod(the rod that connects the piston to the crankshaft). A broken connecting rod would cause free spinning and no noise because it is what moves the piston and the piston is what causes the resistance and noise when it moves. To be sure, though, you may need to remove the engine and open it up or take it into a hobbyshop that does repairs if you don't feel comfortable working on it yourself. If you do decide to work on it yourself, be sure to keep the parts organized. Sometimes I take a piece of cardboard and punch small holes in it with a tiny screwdriver to put the screws in. By each screw or group of related screws, I write what part they go to.
Here's a good guide on taking apart a nitro engine.
http://www.msuk-forum.co.uk/topic/20...-engine-apart/
Tamiya has some good info on their site about the TNX, too...
http://www.tamiyausa.com/product/ite...oduct-id=43530
...including an informative exploded view pdf.
http://www.tamiyausa.com/pdf/exp/43530e.pdf
On pages 8 and 9 it shows how to remove the engine and take off the muffler, clutch and starter housing.
On page 16 it has an explode view of the engine, which should help you out.