"Blowing up" is usually used to describe a severe and expensive failure. It will not actually explode.
If the head gasket is leaking coolant into the cylinder you have a problem. Cranking the engine with one cylinder full of coolant can "hydrolock" the engine, usually resulting in a broken piston or bent connecting rod. You will have just "blown up" the engine.
There are many other types of headgasket failures (recently classified by Stevebfl, I think) that may take a while to become serious. Try a search and you'll find a lot of info.
Experience is something you don't get until just after you need it.