oil pressure was 60psi then went to 0?

Hi Gang,
our Kubota tractor usually has very good oil pressure 60+psi cold and runs at 45+psi.
It has been cold here in western Canada but the diesel motor starts and runs perfect.
This time of year we use it for pushing the snow.
Started the motor as normal and was watching the mechanical oil pressure gauge (we installed) while it was warming up.
The gauge started moving as I was cranking and when the motor started the pressure was at 60+psi and then went to zero . I shut it down and removed the oil pressure relief valves near the pump hoping that they might be stuck open, they looked closed.
The motor is a two cylinder 18hp. diesel- I have the workshop manual but it doesn't show any where that I can see that there would be anything to break in the oil syslem.
anybody happen to have any insight on the lube system on these Kubota.
Is there any chance we sheared off a drive somewhere?
thanks in advance
Tow'd