Normally compression gauges will not register crankcase compression as your measuring the combustion chamber pressure. With a reading of zero compression it can be a few things. Rings that are stuck can cause low to no compression. Blown head gaskets should still produce some compression if your compression gauge is sensitive enough (one for testing small engines and not for autos). A decompressor stuck open could very low compression.
What is making no sense to me is the zero reading as there should something; unless the rod was broken or a hole in the piston which with it starting up again rules this out.
