Hey J,
How long ago was it that you started engine and for how long run ?
Have you checked the dip stick to see what it shows, i.e.- oil, NO oil, water in oil, lots of water. Was there water in engine(checked) last time you started engine? Have to do process of elimination...from afar,eh ! So, what I would start with investigation if there. WHAT condition is battery and connections ? Is clutch disengaging ?
BTW..leaving gas valve on, as you state, is not liable to stop an engine from turning over. However, an engine without sufficient oil and water when run....can overheat and seize. Broken/bent engine internals will prevent turn over. Tell us condition of engine, if known, rebuilt, original, untouched...what ???