Is it blowing cold air or not blowing at all? If cold air, check coolant level first. Check that when you switch the temp control from cold to hot the water valve in the heater hose assembly on the firewall moves. What is your engine temp? Is a stuck open thermostat making your engine run cool? By turning the temp from hot to cold do you notice any difference in temp, do you hear or can you see the temp blend door moving. If all that stuff is ok, remove both heater hoses from the heater core and put a garden hose to it to see if it is plugged. If so try to flush it out.
If you have no fan/no air movement check the fuse, then find the blower motor resistor and wiggle the plug, find the blower motor and bang on it and wiggle the plug. if it doesn't come on put 12V to the fan, if it doesn't come on replace it. If it does come on check the resistor.