Car was blowing cold air and when I got home I noticed a pipe came loose from the header/expansion tank, but car did not overheat, why ?

Probably because the weather wasn't hot and you weren't in slow traffic, and it was only 3 miles or so from the time the coolant got so low that it wasn't going through the heater core.

Put the hose back in place and fill the expansion tank to the hot or cold level with 50/50 water and antifreeze, or just water. If the engine is cold, look in the radiator to see how low it is and refill it. If the coolant level in the block is below the temperature sensor you will not get an accurate reading on the gauge on the dash. Since the heater doesn't heat, I think that's the problem. Check for leaks over the next several days, checking coolant level when the engine is cold. Never put cold water in the radiator when the engine is hot unless the engine is running, or you can crack the head.

Once you lose a lot of antifreeze the temperature gauge is not accurate and you likely are overheating the motor with out knowing it. You have an air pocket in the cooling system. Open the radiator cap and run the motor for 15 minutes to purge the air and top up the radiator with a 50/50 antifreeze.

Dittos to "Bill" - coolant level is low - there is air in the system including the heater core.

The engine warms - coolant expands and goes into coolant expansion/recovery tank. It is supposed to flow back as the engine cools - but with the hose loose, it's lost. So every time you started engine, you lost coolant and there's now air in the radiator, engine, heater.

WIth the engine cool, take off the radiator cap and check level there - think you'll find it is low. Connections to recovery tank need to be sealed and proper coolant restored.

If it did not overheat then it should be fine in that respect but the cause of the pipe blowing off is of major concern.

the cooling system is a sealed system the over flow bottle is there in case it has a leak in your case your car did not over heat so your cooling system has no holes or leaks
on a side note a hose is a rubber tube and a pipe is metal
when you say a pipe came loose it confused me cause theres not many pipes in a cooling system there all hoses

The expansion tank is just to hold overflow of expanding anti-freeze. once the tank is empty and the anti-freezed and been heated fully no more will come out of the radiator - normally not a big deal - the radiator will be 90% full and function. The cold blowing air has nothing to do with the overflow tank.