You have a private heater that has fallen out
If there is a digital display on your heater, please remember the digits(s) that are indicated.
Check the following:
Is the water pressure minimal 1,0 bar (see manometer), if not fill the system up to 2,0 bar. Let any air out on the radiators.
Is 220 Volt power present (any indicatot lamps on?). Check fuses if not.
Can you reset the heater by pressing the reset button? If no reset button you can unplug the heater from the power socket, wait at least one minite and reconnect it again.
If the problem persists or happens often, call your landlord, stating make of heater, the error code(s) and any other information that could lead to fixing the problem.

There is a common heater in the house and you are cold
Are your fellow tennants having the same problem? If yes, see the remarks above.
If you are the only one and you live on the top floor it could well be the system has to be filled up to 2,0 bar. Let all air out of your radiators(s).
If pressure in the system is OK and no air comes out of the radiators, it could mean there is something wrong with the radiator knob. Especially the thermostatic knobs get stuck after a period of inactivity. If you are handy Andy remove the knob by loosing the screw and by gently pulling backwards. You now see a little pin that goes in the valve. Push this pin a few times with the back of your screwdriver and mount the knob. It should be fine now. Do not force anything, if you envisage a problem call the landlord.

It is not freezing but the radiators do not get warm enough
This is generally caused by too much air in the system. Let out all air and refill the system if the pressure is too low.

There is no supply of hot water but the heating is working
Have a look at the shower head or the warm water tap. In Haarlem and its surroundings the water is extremely rich of chalk. This blocks taps and shower heads regularly. De-chalking is a responsibility of the tennant, buy a bottle of f.i. "Anti Kal" or equivalent in a supermarket and follow instructions as indicated.

Never change settings on the heater or on a (if present) regulating unit. Never experiment. If you cannot fix a problem the way explained above, call your landlord.

Filling the system
The Manometer
The Error Code
The resetbutton
Letting air out of the radiator
The thermostatic knob
All types of fuses and safetty switches