First thing to try, IMHO, is to turn both sides up to "Hi" and see if you get worm air from either side. That will rule out the outside temperature sensor.
If you get heat one side only, then likely the arm has come off the blending flap for that side, as suggested. If it's cold both sides, then maybe HBV or its' vacuum actuator, or the heater matrix needs flushing, or you're unlucky and both arms have fallen off.
The HBV should be open with no vacuum applied, so run it with the vac pipe removed from the HBV to see if it heats up then.
You can also remove the trim above the driver's feet by pulling out the 3 circular plastic clips and you'll see 2 silver pipes on the left hand side of the footwell that feed the heater matrix from the bulkhead. See if these are getting hot (too hot to touch). If so, you have coolant flow to the matrix.