The first thing to check of course is the coolant level to make sure it is full and that there are no pockets of air in the system that could keep the heater from working. If the coolant is low, then we will need to pressure test the system and repair any leaks before we can focus on the heater problem.
If the motor is getting up to normal operating temperature check both heater hoses when the car is hot. They should both be hot to the touch where they go through the firewall If the are not both hot and equal, the heater core is restricted or plugged. Sometimes they can be flushed out with a garden hose or compressed air, if they are too bad they need to be replaced. The only other thing would be if the hot water control valve in the heater hose and is shutting off the water supply to the core. This can be tested by taking it off and seeing if you can blow through it. Also if the core is okay, the heater mode doors could be not opening to let the heat into the car. You can check this by setting in the passenger seat and running the temp control lever back and forth from hot to cold and listen under the glove box to see if you hear the door open and close. If not you could have an unhooked or broken control cable, a problem with the control panel or in the mode door itself.