Hi i just bought a 2004 kfx 700 and have some problems with it please help?

it cranked up and ran good for about 20 seconds but the battery is dead and i jumped it with a car and when i took the cabels off he ran sluggish and i gave it same gas and it started backfireing like a pop pop and i cranked it up abut three times and now the lights will not come on and will not do nothing like the battary is dead help please it set uo for a year and there was nothing wrong with it when parked and was inside a barn not in the weather