need more explanation - did you put the head back on and fill it with water, or is the head still off and this is the oil you thought didn't have any water that's been sitting since december?

it's possible that the water had been sitting in the bottom of the gearbox case the whole time - the dipstick doesn't go all the way to the bottom

If you did not drain the coolant before you pulled the head off
I will take a guess and say the coolant has got into the gearbox / sump
when you lifted the head

Mark

It's got into the cylinders, then past the rings into the sump.
Drain it all off and refill the oil- if you didn't try to start it since, she'll be right mate.
A new filter would be a good idea too.

It would have taken a while for the coolant to fill the cylinders. Usually the engine will run rough and stall then wont restart. Then the cylinders fill up. If the oil is not milky then my bet is that it filled the sump after the engine was stopped. If the engine was turning over ok, then trying to start it for a few minutes after it stopped should not have caused any major damage. The term Hydraulic lock is used when a piston tries to compress fluid and this is where the damage occurs.

Check out google images.

Milky oil. The oil looks like it has been mixed with milk in a blender.

Hydrolock. Main damage is bent rods. You should be fine because it didnt go bang.

Just drain, flush, filter change.