Changing the fluids is not going to solve the problem. It sounds like the leak is so bad, a cylinder filled with coolant. That is called "hydro-lock". Liquids cannot be compressed, so the piston cannot move, and the engine will not crank until that liquid leaks out somewhere, which in this case sounds like it was past the piston rings and into the oil.
The head gasket is a big job, but it is important to be aware that antifreeze will melt the soft outer layer of the engine bearings if it is allowed to sit in there for too long. That will more than double the already high cost of repair.
To verify this, remove the spark plugs, then try to crank the engine. If you see coolant shoot out of one of the holes, the head gasket is leaking.
Thursday, May 4th, 2017 AT 8:19 PM