my polaris 400 sport is a 2stroke and is in relatively good condition. when i took it out in the snow it worked very good for about a week. then i went to start it and it would start but it wouldnt idol. i revved it for awhile and it seems that something is stuck in the engine and wont come out. but when i let go of the gas it screeches and turns off. someone please tell me whats wrong  

It could be a number of problems here. The primary focus should be on the carb. A clogged vent hose, debris in the float bowl or a clogged pilot jet.

Then there's the screech you mention, that sounds like a sticking throttle. Does the throttle snap back to the closed position when you let go?

I'd pull off the carb after checking to make sure the controls are moving freely and not binding anywhere.

A more serious problem could be you siezed the piston and it'll start but when the metal heats up, it sticks (siezes) again. The "it seems that something is stuck in the engine and wont come out" leads me suspect this. If the carb and throttle checks out okay, you might be looking at a top end job at a minimum.

Good luck,

Bryan