Radiator not filling up after it was removed and re-installed for plastering wall.

I suggest it would be a good idea to isolate the radiator via its lockshield and thermostatic valve, then drain and remove from the wall. Now the radiator is out of the way, it would be a good idea to place a large container under each of the valves in turn and carefully open them to see if the water flows easily and there is no blockage. On the assumption that all is okay then remove bleed plug from radiator and check there is no blockage here. Replace radiator, tighten joints and with the pump turned off, open valves one at a time. You should hear a small amount of water enter the radiator from both valves. Open bleed valve to confirm air is being expelled. If the problem still remains than suspect a "blocked cold feed" pipe. This can easily be confirmed by attaching a hose to a drain point, place the hose outside and open drain valve. You should experience the float valve in the tank in the loft drop down to replace the water as it empties from the system. Immediately the float valve opens close the drain point to prevent a low water level occurring. If the valve did not open after only a minute of two than its likely you have a blockage on the cold feed pipe into the system. This is quite common particularly on systems that have a degree of muck inside them. Its a simple job for a competent heating engineer to identify and sort out.

blockage plaster in valves,or thermostatic valve jammed shut.