Possibility #1 Lifter out of it bore...
Possibility #2 Stuck oil pressure relief valve...
Possibility #3 Broken drive...
If you want to start the car with the gauge line disconnected I'd put the hose in a clear bottle.. I had one of those hoses break under my dash back in the 70's.. Big mess.. It may be a small tube, but at 60 psi it can move some oil...