Head gasket? can it happen instantly ?

It can but i think probably most develop much more slowly and give warning signs. If you think your gasket may be leaking and you are losing water pat the radiator header tank cap take the car to a garage and ask them to do a Combustion Gas Leak test, it is quite cheap and only takes a few minutes.

Usually there are some signs of the failure, such as use of excessive coolant or coolant mixed with the oil in the crankcase. But if the car overheats is can go instantly.

Both. Sometimes there are gradual tell-tale signs such as an external oil leak, or coolant loss, or oil in the coolant. Or it can fail immediately causing a misfire or all of the above.

usually there are signs before it will go out as in every so often your car can start to overheat, or if your coolant levels are low, or if your head gaskets haven't been changed in so many years or so,or if your head bolts are no tighten to company standard. It can be numerous amount of things.

It can fail instantly. I would check the simpler stuff first that can lead to a loss of power. i.e. fuel filter, coil pack, air filter, timing belt, etc.

they can go just like that without any warning ! you can be driving along and all of a sudden white smoke will blow out the tailpipe !

Both are possible. Usually though it starts with a small leak which gets bigger

Mine did at 6.45am in the middle of nowhere.

UK