The first thing to check is that theres water in the system! No heat is a sign of this and it has to be remedied quickly before your head gasket blows! If theres water in there and no leaks, then its either
an airlock in the system
Thermostat
or Heater matrix
I know this posting is too late this time, but hopefully
it will help others in a similar situation
Andy Barber, November 2009