If the fan is blowing, but no hot air is coming out that means that no hot water is making it into the heater core.
I can think of 3 reasons for this, each with a different solution.
First, and easiest to check: make sure that there's enough coolant in the car. If it's low, there may not be enough fluid to circulate through the core.
Second, the valve that opens to allow hot water into the core could be blocked (or the core itself could be blocked). You may be able to check this by undoing the intake and return lines from the core and seeing if you can get water flowing, perhaps by putting a hose on the intake. Be careful about applying too much water pressure, though - you don't want to blow up the core and end up with a very wet interior and a very expensive repair.
Finally, if you have an automatic climate control system, it could be a sensor that thinks there's too much heat going though the system, so is cutting off the flow of how water into the heater core. If it's an old mechanical system, the actuator may have come loose, so it's no longer operating the valve. If it's a sensor or other electrical gremlin, you'll probably need to take it to the dealer, but if it's a mechanical system you can probably trace the link & see if the valve is operating properly.
