I have an audi a4 1.8 se 1998 the heating system

check the coolant level and start the car if you get normal temp which means from 60 and above check the two hoses under the hood if both are hot if your car is has heat reguletor on one of the hose check for current if you have curnt then the solonoid is not working but if you have the electronic system let me know.

Its very probably the heater core underneath the dashboard. This has the connections and valves that draw hot water from the engine cooling system. If this is not working then the heating will not work. This needs to be checked out by a mechanic as its pretty tricky to fix and may need to be replaced. Check that the fuses are good first though as it maybe something as simple as that.