could be one of several things.
--if it quit after changing oil or setting a long time, the pump may need primed. take off the 1/4 inch pipe tee at right rear corner of motor and pump oil inside, then reinstall and start.
--if it stopped after changing the oil filter, maybe you have the paper filter instead of the cloth filter. or maybe you left the 1/4 inch steel tube out of the filter?
--if the motor is very old and never rebuilt, maybe it just has a major loss of pumping from wear in the bearings..
--this is how it works. the pump sucks from the crank case and send oil down the center of the cam shaft. holes thru the mounting bearings lube the crank and cam and oil sprays on the bottom of the pistons and drips down onto the connecting rods. outside you can see the small tube that runs to the top end (head) and lubes the rocker arms and valve guides. take this loose and start up. see if you get any dripping. maybe you have "flow", but not much pressure. you normally only get 12 psi on a good day. lots of the small tractors only have 4-5 psi and still run o.k. how loww does your gauge go??
--as i said the oiil goes from pump to the motor. about 15% is blead off and goes thru the oil filter and you see pressure on the gauge here, then drains back to sump. if the filter is bad, or tube internal is gone, you might get too much oil draining thru it back to sump and not seeing good pressure. iniside the filte the 1/4 inch tube has two small 1/8 inch holes to feed the filter. if the holes are small (you could plug for test) then you gain a little more pressusre to the motor. several ideas to try.