Take the drain plug out and drain oil, measure quantity. Look through the drain hole to see if the screen fell off or if it's loose (coat hanger with small bent hook to see if it moves) Did you spin the pump with the filter off to see if oil came out? Take a can opener to the filter to see if it collapsed inside (if it's orange, there is a VERY high possibility of that)
On my '91 Ford E350 cube van with a 460, I hooked an oil container full of ATF with a line to the oil pressure sender port and added a little air pressure to back blow the system and I got globs of filter material from the Fram I took off. I had opened it up and found it had collapsed leaving me with zero oil pressure (my photos disappeared when my computer crashed and Photopoint.com, my online album, went bankrupt and folded ) Then I used an external oil filter setup to bypass the filter (joined the lines together) and ran another couple quarts of ATF through with the oil pan drain plug open and spun the oil pump backwards with a drill. More globs of whatever came out of the drain hole, as did chunks of the original plastic tipped timing gears (someone put new steel ones in without cleaning the pan) so I used a coat hanger to dig out as much as I could (oil pan is 23 hours labor to do plus you have to take the motor apart to get it out to do it)
I strained the ATF and flushed the pan first with engine flush pumped into the top of the motor with the valve covers off (not into the oil gallery) with my parts washer till globs stopped coming out and I couldn't fish anymore out. Then I poured the ATF through again and put on a new super fine filter. After using the drill to pump the system up again, I assembled the motor enough to run and make sure oil was coming up to the rockers before putting the valve covers on. I cleaned out the oil return galleries too, which were full of crud, even though the log for the motor showed oil changes every 3500 miles (with the orange filter)