Q: Car turns over, but will not start. The accelerator pedal produces NO change in engine performance.

This is my backup car. As such, it sat for 3yrs. This summer, put new battery in dropped sea foam in oil & gas, & a bottle of iso-heat when I filled up the 1st time. I've driven it regularly (at least every few days) since mid-Sept. w/o so much as a pause--this little Mazda is rough--she's NOTHING if not reliable. proto was then parked for 4days, i try to start it, & it was ROUGH (already kinda rugged cuz the exhaust just hangs from the manifold back to the muffler) so i let it warm for a few (probably TOO few, as it was only 12°F) I got ~1/4mile & noticed it was sputtering bad & not responding when i stepped on the gas. As i pulled back into driveway, she rumbled until she sputtered herself out. Let her sit overnight & now won't start!! Turns over, but sounds like it will, but won't STAY started. Gauge shows 1/2 tank so pretty sure not fuel pump, sprayed starter fluid in injectors & cranked, but no change...

TRIVIAL INFO: actual tacometer reads 398,000 orig. Engine, but new head