Please help!! no oil pressure after oil change!!

the engine is full of gunk ( carbon, maybe some old gasket material) someone didn't change the oil enough, and the oil pick-up got clogged up, and I would suspect the engine bearings are now damaged, but a aftermarket oil gauge to monitor engine oil pressure, you should have atleast 40 psi of oil pressure, and the max should be 55 psi, if your engine doesn't have atleast 40 psi, then the engine should be rebuilt ( including having the crankshaft turned and new cam bearings, as well as the main & rod bearings, new timing chain, rocker shaft cleaned etc