Is the fan running, but no heat? First thing I'd check is the coolant level. Assuming the blower fan is working there only remains two things. Either the blend door is not operating and set all the way to cold or the heater core is clogged. Since you seem to indicate the heat stopped abruptly I'd think the blend door is not operating. If you move the heat control all the way to cold or hot do you hear the door move? If the core is clogged or partially clogged you may be lucky enough to only needing to flush the system. Prestone SuperFlush works well for this. If that doesn't work and the blend door is okay, you may need to replace the heater core.