hi Emma,
you turned the knob to test the pressure relief valve
which will drain the water from the heating circuit. If you don't have the charging key or the black socket to put it in it might not have been fitted (optional).
you need to look for a silver braided hose joining 2 pipes (one 15mm and one 22mm )usually underneath the boiler. there will be a black tap at one or both ends, if you turn the taps inline with the pipe (on position) you should hear water running in to fill system, keep an eye on the pressure gauge and when the black needle reaches 1-1.5bar turn off taps.
reset boiler and should then run ok
Martin
October 2015