Cannot fill Vaillant Turbo Max plus 824E ?

No pressure indicated on gauge and yet Radiators appear to be heating ok. Switching on Hot water Tap brings on Flame light, but goes out. Hot water supplied briefly, after usual delay but then goes cold. Have tried to refil the system using the two Taps, but nothing happens. Have done this sucessfully in the past. That time, I could hear the water re-charging the system. Not aware of any leaks in system. Could this be a faulty Tap or a valve elsewhere in the heat exchanger circuit? Has anybody else out there experienced this fault? Any help on this matter would be greatly appreciated as this is for my Mother who is a poor pensioner...