Why won't my john deere sabre 14.5 OHV 32 or 36 inch cut lawntrctor start? I have adjusted the valves once because iwas told to try it,and it started I mowed my lawn with it then a week later I mowed my lawn again and after that it wouldn't start again. I was told that the head gasket was the problem, but wouldn't it still run?

If the head gasket is leaking then it will have low compression .
I would recheck my valves and make sure they didn't come loose.
Luck

I agree, have you done a compression test, I sure would