Okay. Be very very careful here. Don't run the engine until you find out why this is happening.
First off, try the obvious. It could be a faulty gauge or sending unit.
Remove the sending unit and hook up a mechanical pressure gauge where the sending unit was. This should be on the front of the engine on the drivers side.
If you start it, and have oil pressure reading on the mechanical gauge, you are very lucky and have no internal engine problems. It should have 40-60 psi at 2000RPM.
If you do not have any oil pressure as soon as you start it, SHUT IT OFF.
You could have a faulty oil pump, a twisted or broken oil pump drive shaft, the sump (pickup tube) could have fallen off inside the pan, or maybe the engine has spun a few bearings.
If it is an oil pump problem, and you don't run it or drive it, you may be able to salvage the engine and just make moderate repairs.
If you have oil pressure, then just try a new sending unit. If this doesn't work, maybe the gauge or wiring to it needs to be replaced/repaired.
Do not drive it or let it run for more than 7 seconds until you know for sure.