I assume you mean it lost oil pressure?

Are you sure it really has?  Did the lifters start clattering and you
heard other ugly noises or is the oil pressure gage or idiot light
telling you no pressure?

On those engines isn't the oil pump spun by the distributor?  If so, and
if it really does have no oil pressure but it still runs maybe the shaft
going into the oil pump broke?  I know some engines have shear pins on
that shaft and maybe the pin sheared, although the ones I'm familiar
with  that will stop the distributor from turning too.

If it really has no oil pressure and you replaced the pump it has to be
either the pickup has a leak so it's sucking air instead of oil, the
pump isn't turning for whatever reason, or there is a massive internal
leak that is bleeding off all the pressure.  It could also be a massive
external leak, but I imagine you would have noticed all the oil on the
ground!

Did you or anyone do any kind of work on the engine shortly before this
happened?

Jeff DeWitt

You need to get a mechanical gage to test to see what the real oil pressure
is.

Jeff

If you pulled the pan, then pull the distributor and see if the pump
drive shaft, sheared the pin holding it to the drive gear.
  You might have checked to see if the gauge was working ok before all
the grunt work, though...