The first thing that springs to mind is an airlock. Why one should suddenly
crop up is another matter especially as you indicate that you haven't lost
water. Have you tried bleeding the system? I'm not familiar with 306s but on
my 1.9TD Xantia and on my wife's 106 there's usually a bleed valve like the
dust cap on the tyres somewhere. You may also need to make a header tank to
get the water level up above that of the bleed valve.

Secondly it could be a sludged up heater matrix. How old is the car and has
the water been changed at the specified intervals? If bleeding doesn't help
then you may need to remove the heater hoses from the engine and run water
from a hosepipe through them and then refill and bleed. I had to do this on
the 106 in November. Well sludged up it was, like rusty mud.

I have also heard of the heater control flap not moving but not having had
any experience of this I'll keep quiet as I don't know how your heater
control works.

My 1995 405 had a tiny fan in the dash, blowing interior air onto a temp
sensor. It used the temp read from this sensor, to decide whether to blow
hot or cold air into the cabin.
These fans were known to die - and if they did, the heating system assumed
the cabin was already warm (or cold) with the same symptoms you describe.

Also, note that the required temperature from the blower, was obtained with
a motorised flap that chose between hot and cold air (or a mixture). Again,
if it got stuck or the motor died, you'd be stuck with hot/cold/mixed air.