How do you know it has stayed blocking the hole? (picture)? If that is the case, unless you can either refit the broken plug or fit a new plug, then you will only be running on 3 out of 4 cylinders (or 4/5 whatever) because you will have no compression, this will lead eventually to HG failure or piston ring failure.
It could be the plug has got carbon build up on it and that's why it didn't come through the hole. Or it could be that you broke it when you pulled the plug out and pulled it away before getting the whole thing out.
If you can confirm the plug is not in the same hole as the inlet valve - that it has its own chamber, then you can safely push it back in.
If it is in the same chamber as the inlet valve then you should try to refit the broken plug, (you could angle grind off the bit below the thread to make it shorter.) and hope that it remains stuck where it is, because if that enters the valve gap your engine is knackered. If it were me I'd take the shortened plug out again (straight away) to see if it has dislodged the plug end, and then stick a magnetic tip in there, if you have access shine a torch in to see whatever you can see. or if you're really clever get a small LED bulb and lower it into the chamber to see inside. I suspect a torch will be next to impossible to see into the chamber, it's hard enough on petrol cars through the spark plug hole (and they're bigger holes).
No, I doubt the compression will blow it out, if it is loose inside the chamber it will at most rattle about, if it is stuck in the hole then the air will just escape around it.