Pull the plug and spray WD-40 into the cylinder, ALOT. Come back tomorrow and see if you can turn it over by hand. If so, congrats it's NOT galled to the cylinder. Tear it down and inspect the cylinder, looking for deep scratchs, cracks, flaking, etc... (if you have a question ask us, post pictures if you need an assesment of cylinder condition). If it's good, go down and grab a topend kit, race home and put her back together. Might still make the track
If it's galled to the cylinder walls, that sucks big butt
I did that on a CR500 I owned and had to purchase a new cylinder. Piston and cylinder decided to become one.
*** EDIT ***
Forgot to mention, inspect the connecting rod as well, looking for bluing around the bearing / wrist pin ends. Also, once the cylinder is off and the piston is off the rod, rotate it by hand (keeping tension on the rod) to ensure the rod bearing is still good. If it feels bad, it is bad