Heater suddenly stopped heating

[syncronized]

Started the old JX diesel up today & after a few miles noticed there was no heat coming out the vents, just cold air. The blower motor works fine and I got someone to move the hot/cold control & looking up from under the van I can see the valve lever in the heater pipe moves ok. Coolant level is fine too. Any ideas why it should suddenly stop working?

[kevtherev]

blocked matrix, air lock, low coolant, faulty valve, kinked hose, blocked hose ...
follow the Baxter bleed method and see if it helps shift any air lock
blow through the matrix to clear any blockage
check for kinky pipes

[Aidan]

plastic lever on valve can strip so looks like it's moving but only the arm is moving the inner part is stuck fast

[syncronized]

Thanks for the replies. I'll check the pipes, valve & bleed the system & see if that cures it.