Depending on how long it sat, the oil may have drained back to the pan and the pump may have lost prime. It will reprime (unless it's got too much wear), just slowly, since it's designed to pump oil, not air, and it has to move air to get the oil to suck up to the pump again.

You can do mech without a gauge - just remove the sender and block the hole with a finger and crank it and see if you irresistable pressure. You should not be able to hold cold oil pressure with your finger.

Not responsible for advice not taken...