Sudden loss of pressure?

I have a Vokera Linea combination boiler. It's been working fine then suddenly flashed 04 error (low pressure) The pressure gauge is down to 0. When I turn the fill knob I can't hear water. I have checked for leaks around the flat but no obvious evidence. I have bled a couple of radiators but there doesn't seem to be any sign of air.
Can anyone tell me why this has suddenly happened and what else I can try before I call out an engineer.
Many thanks