Heater water valve stuck shut, mechanical linkages to air flow
selector problem. Feeling the flow/return pipes may give some clues.
Bleeding the system won't hurt.
Other possibility hose disintegrating inside.
I usually detach and extend the return hose, then with
the heater valve open, see if hot water can be returned
back to the radiator filler.
If the hot water isn't circulating then you know where
to start looking.