Hot water OK but no radiators hot (or vice versa)
This will work for Fully Pumped systems, whether you have one 3 port valve or two 2 port valves.
To get you warm, turn the HW thermostat (on the cylinder) right up , say to 70, and the thermostat (eg 1-6) on the boiler down to 1. Then put the CH valve lever ( or 3 port valve lever) in the hooky place which holds it in the MAN position.
Put the HW ON at the programmer.
The system will then try to make the HW hot, but fail to get hot enough because of the stat settings; meanwhile your rads will warm up.
Thermostatic rad valves will still work but not the wall thermostat.
If there’s a little bump on the top of the motorised valve head (only for Honeywell valves), it means you can remove the head (actuator) from the body to replace it without getting wet. The mechanics of the heads do wear out and jam, but you can just change the motor. (Synchron motor is sold widely).
Sometimes it’s easier to take the head off to get it into an easier position for you to fiddle with the two wires to change the motor, then put the head back when done.
You can use an ordinary flat bladed screwdriver for the motor and head screws, but for the fairly recent ones a magnetic T15 Torx screwdriver works better.
If you change the motor with the head off, wiggle the lever to help seat the motor .
If you notice any moisture leaking from the valve body shaft, it probably will have got into the head and rusted it, and possibly the switche(s) as well. You need a whole new valve, and that means draining the system.
Isolate the mains power from the heating system before you start any work.
*If you have two, two port (pipe straight through) valves and have “lost” your hot water, that valve may not be opening. If you latch the lever on the valve in the “Man” position, the water will heat up whenever the CH is on.