Worcester 28CDi boiler will not fill?

My Worcester 28CDi combi boiler stopped providing hot water this morning while I was in the shower.

Looking at the boiler, the pressure gauge is reading only 0.5 bar (operating pressure is between 1.0 and 1.5). I tried to refill it, but no water appears to be flowing into the system.

To refill, I have:

(1) inserted the internal filling loop key into the relevant space, and turned it to the closed padlock symbol to lock it in place. The key is fully inserted.

(2) opened the (in this case grey) filling knob as far as it will go.

These are the only steps mentioned in the manual for refilling. However, after doing this I cannot hear any running water, nor does the pressure reading move at all.

To try and work out what is happening, I removed the fill key and opened the grey knob again. This resulted in water flowing out of the hole where the key should be inserted, suggesting that the grey filling knob is fine and allowing water to pass. But when I re-insert the key to try again, opening the grey knob does nothing, with no flowing water and no filling taking place as far as I can tell.

Elsewhere in the house, all of the radiator valves are open, as required in the manual.

Any suggestions on what is stopping the boiler from filling?