Most likely, the thermostat is no good or in backwards.
If it was in backwards, the heat sensing pellet would always be on the cold side and would never open. This or a bad thermostat never opening could fry your motor from overheating without the coolant getting hot. If you pull the dipstick, can you hold the oily end or is it too hot. Does the motor crackle when you turn it off? Does the head feel extermely hot. Be careful of the exhaust manifold, no matter what, it will be hot.
If the motor is very hot, you are in extreme risk of ruining it. Usually you warp the head and blow the gasket. Things other than a thermostat, like a bad water pump or some other blockage of coolant flow could cause this same condition.
If both the motor and the coolant are both cool, the thermostat is stuck open. Pull it out and test it in a pan of hot water. You could also try blocking off the radiator with cardboard and see if the coolant gets hotter.
Obviously, driving without a temperature gage is risky. So try to determine if both your coolant in the radiator and your engine is cold, or if your engine is hot, but your coolant is cold. If the engine is hot and the coolant is cold - don't run it until you get it fixed.