Head Gasket Leak

I want to start this out by saying I know nothing about vehicles....I originally brought my
vehicle in for an oil change and to check on the air conditioner since it didn't get very
cold. That night after a $200 bill my air conditioner still didn't work properly and my
check engine light came on and shortly after overheated. The coolant was completely
empty when I checked. I immediately replenished the coolant after finding it was
empty.  I took the vehicle back in the next day and was informed I have a leak in the
head gasket. Now my vehicle can't be driven more than a mile without overheating. I
feel the whole situation could have been prevented had they not been negligent in their
job. Does anyone think the repair shop could have caused the head gasket leak? The
repair shop denies that they caused the leak.