Chances are you filled the radiator then instantly started it, water did not have a chance to get to the block, the block got hot, water then circulated to the block, and there was instant steam, which was why it all came out.
I suggest refilling the radiator and let it set for several minutes. The thermostat quite likely has a bypass which will allow the air to escape the block so that water will enter the block area. So, refill the radiator, let it set a few minutes, refill again, let it set. Repeat this process until the normal amount of fluid is in the coolant system. Put the radiator cap on in the lose position, so that pressure will not build up, yet the cap remains securely on the radiator.
Start the engine and let it warm up. Be careful. Many people have been injured and killed by scalding hot water gushing from a radiator.
If it still blows back out, look for blown head gaskets, cracked block or cracked cylinder head. These items will also cause overheating, and cause the system to puke out it's water as though the radiator is cracked.
