gradd

i just bought a 98 arctic cat thundercat 1000 from my boss and once i got it i put new plugs and fuel filter and cleaned the carbs.so i took it out today and was out out for mabe 10 minutes and it lost all power and then died. it wouldent pull over so i went to the clutch and then turned it over but herd the piston grinding in the cylinder so we popped out the plugs and the middle plug had piston fragments all over it so i towed it home and when i took it apart the piston had a hole in it and the rings were pritty much welded to the piston. the other two pistons are fine and i was wanting to know what happend to the middle piston? and why it happend? the sled just got a $500 tune up, im thinking they dident do anything to the mechanicals they were supposed to go over the whole thing. there were no new plugs and the anti freeze was not topped up just new sliders and belt. anyhelp would be great! and where can i get stock pistons for a good price? and how much will it cost aprox for cylinder rehoning?
thanks Gregg