That happened to my girls car, her woodruff key on the crank pulley broke and the pully lost alot of tension. I fixed it once but the little square on the woodruff key was broke on the shaft on the engine part, it messed up again and again so we just swapped the motor.

Take all your belts off, take the drivers side wheel off, and see if you can move the crank pulley by hand while the car is in gear. If you can, it may be the same kind of problem.