help! 98 kx 250
 

i recently bought this bike used it ran great i was able to get out riding with it a handful of times ( roughly 16 hrs) and i was in sand pits, and near the end of the last day the bike just died, i was in second gear everything was fine went to give it gas and it just died. it wont kick over now. i replaced the spark plug and still nothing, my friend told me it might be the piston but besides that what do you guys think?! this is my first bike just learning how to ride it i really want it fixed to finish out the season strong! your help is appreciated