You have done what is called hydrolocking your motor. Basically, a cylinder or more than one, has filled up with water, and since it doesn't compress at the same ratio as air does, it siezes the motor. It can be temporary or catostrophic.
First step, pull all the plugs and see if it turns over with the plugs out. Stand back, cause if it does, it will spray water everywhere. That is a good sign. Reinstall spark plugs, fire it up and let it run for a little while to get all the remainin water vapor out of the intake and exhaust. Change oil and air filter.
If it doesn't turn over, pull the starter and bench test it. If the starter works fine, then don't bother with the oil change, you have something in the rotating assembly, probably a rod that is bent, siezing the motor. This is the catostrophic portion I mentioned above. Time for a new motor or at the very least a rebuild.