My heating system has all of a sudden statrted.

First of all make sure that the coolant level is correct.

if still no heat, remove the blend door actuator (under dash). turn the "arm" that the actuator fits onto. this will open the heater door. if heat now is available, replace the actuator.

this link will help.

http://www.youtube.com/watch?v=D_GZNbP4mwo

Before removing anything I would also make sure both heater hoses going into the heater core at the firewall are getting too hot to hold when the engine is warmed up?Also watch if the blend door is operating.