If the temperature gauge isn't moving much from the lowest reading, or the car runs poorly for more than a few minutes on a cold day, the cooling system may not be working properly. There are a few potential culprits for why your car engine coolant may not be heating up:
Coolant Level - The first think to check is the coolant level! If the coolant is low, there may be air in the system, which will lead to localized hot and cold spots. Typically the gauge needs to be in the coolant to read properly.
Air Lock - If you have had a coolant leak, or have recently drained and filled the system, then you may have air lock. When this happens an air bubble gets trapped in the system preventing the engine coolant from circulating correctly.
Gauge/Temp Sensor - Is the gauge actually working? Is it reading correctly? Modern cars have an electronic coolant temperature sensor connected to the OBDII computer which informs the temperature gauge. These sensors can fail. Often they will be flagged when you read the OBD fault codes.
Thermostat - A faulty thermostat can cause too much or to little cooling. If it's stuck open, the the engine is going to take longer to warm up because coolant will circulate all the time. You can remove the thermostat and test it in a pan of almost boiling water, it should open just before it reaches the boiling point, then close as the water cools. If doesn't move, then replace it!
Fixes: The only way to fix a faulty thermostat or temperature sender is to replace it. If the coolant is low, or there is an air lock (and also after replacing any faulty parts) you need to fill the system properly. To do this, set the heater controls in the car to maximum heat, remove the radiator cap (or remote mounted coolant pressure cap, sometimes on the overflow tank) and fill to the correct level. Now start the engine without replacing the cap and idle for a few minutes. Watch for the coolant level to drop as the thermostat opens.
Keep topped up to the max level with the engine running. Squeeze the top radiator hose to help air pump around the system (taking care to avoid any moving parts, in particular the radiator fan, which could come on suddenly without warning). Between the engine heat and the water pump, all the air should be forced from the system. Once full and warm, replace the cap and test drive.