1999 Pontiac Grand Prix blown head

i had my car checked at auto zone bc the service engine light came on with no prior problems except the blower on the fan wouldnt work on settings 1-4. so they said change the spark plug. did that and died 100 ft down the road and friend says its a blown head gasket. is there an exact way to check thats what the problem is before taking it to a shop? would the blower not working properly cause water to get into my engine and blow it? i just cant afford to spend $500 is thats not the problem. and another ? is how do you tell what sunroof motor you have and is there a way to tell if its an after market one since i bought the car used?