'95 old cutlass cierra

i have a '95 olds cutlass cierra v6. i had the oil changed at a lube center and then 2 weeks later it died on the highway when i got to checking it showed there was no oil. the car wouldnt start and a couple of days later it was showing that it was about 2 quarts overfull on oil. i think the motor is locked up. we drained all the oil out and put in a bottle of marvel mystry oil ( i had a friend use it on there car when they ran it out of oil and locked it up and it worked) plus a quart of oil, i let it set overnight but it still wont start. what else should we try. help please