I suspect the return side of the pump has drained out so you'll need to prime that side of the pump. The feed side of the pump hardly ever needs priming as it is gravity fed (as victims of wet sumping know only too well!), however the return side will drain out dry and there is no automatic way of it replenishing itself. This is my method.... make sure there is some oil in the sump (tip a couple of cups down the rocker box via an open tappet inspection cover). Take the oil pipe off the return side at the crankcase spigot. There are 'F' and 'R' stampings on the crankcase. Take the spark plugs out, engage any gear and rotate the rear wheel backwards whilst squirting oil into the spigot. I actually use an old oil pipe cut off at about 3" and screwed onto the spigot. You should see oil being drawn into the pump. Once you have 'lost' about 20cc into the pump it should be primed. Check by rotating the wheel forwards and watch oil exit from the spigot. Fingers crossed, job done! Reconnect the oil pipe (obviously!).
....Another, less sophisticated, method is to fill the oil tank right to the top and leave it overnight so the oil can run down the return pipe via the exit hole at the top of the return pipe. Hopefully, by the morning, enough oil should have found its way into the pump to prime it.
Both the above methods have worked for me.