Pull both covers and check for a loose rocker and (bent) pushrod.
If a pushrod is somehow forced out between a lifter and rocker, the cam will knock the lifter up out of it's bore and you'll loose oilpressure.
There will still be a weak flow of oil near the crank but no pressure higher up or at the gauge.

Has happened 2 or 3 times to me over the years.

This, or the oil pump's pressure control valve is hanging open. Same deal, some flow is present, but hardly no pressure.