Worcester 24i pressure dropped to zero?

Hi

I "bled" my radiators thinking I was doing a good thing. However afterwards I noticed that I was not getting any hot water through my tap. I looked at the boiler and noticed that the black dial on the pressure gauge was stuck at zero (whereas I think it is supposed to be between 1 and 2).

The last time a heating engineer was out at my flat I think he told me to turn a particular screw a quarter turn to fill the heater again. However, I tried this and nothing happened.

There is also a flexible hose hanging down with a white kind of plug on the end with a screw indent but I'm not sure if this has anything to do with the pressure.

Anyway, any help advice would be brilliant - my flat's cold!

Cheers

Graham