backfire kill my grizzly !

hello, my name is Brandon and I've worked on small motors a lot in the past but nothing like a four-wheeler...
i just bought a Yamaha grizzly 600 4x4 second hand for 1100 bucks in great shape the guy told me the top end was rebuilt new stater and starter etc. i really have no clue if the top end was or wasn't rebuilt but there was a new carb and starter. i seen the gas line was leaking gas so i replaced the gas line put fresh gas. once i got it to fire up and run there was issue of backfiring. well i jump on it and slowly pulled the grizzly around my house to park it and as it was idealing and me giving it gas it let out one big backfire and it looked like the flame came from the muffler where you mount the pipes to the head, my grizzly cut off and i tryed to start it and nothing ! my grizzly wouldnt start back up all it would do is crank over ? today i tryed little starting fluid and nothing ! i havent checked to see if i was getting any spark to the plug and most time thats a good starting point, i did some reading and few others had the same issue im having, what could a backfire do to couse my atv just to do what its doing ? i was under the terms that it might be my woodruff key that may have broke is that possible ? please help.
is there a check list i can rule out the possibility of what needs to be fixed ? thank you