Take the backplate off, and use a flashlight (if necessary) to look down inside the crankshaft for any signs of dirt. Also, take the carb off the body, and again check for dirt, flush out with fuel or wd-40 or carb cleaner (my favorate).

If you see dirt inside the crankshaft or under the carb, then you should take the engine apart, since the dirt almost certainly made it in to the bearings, and you'll want to flush that out or replace them if there is any doubt.

I saw an engine (not one of mine) that had dirt all the way though the crank, in to the crankcase, and up in the transfer ports, and in to the combustion chamber. The guy who planted it never pulled back the throttle, and I swear that engine tried to keep running underground.

On the other hand, I've burried engines (combat matches result in plenty of lawn darts), sprayed them down at the field with carb cleaner or WD-40, fired them back up and flown them again that day with no problems. Just make sure you close the throttle before impact, that goes a lot way towards keeping dirt out of the inside of the engine.