no water pressure on vaillant hot water unit?

have had to remove central heating units to lay new floor .When put back there was no water pressure shown on the Vailant hot water unit and an f 13 fault showed up.I have water from both the hot and cold taps but no hot water from the hot tap.What do I have to do to regain a pressure reading on my Vaillant system.There is gas in the system as I can hear the gas burner however it shuts down within minutes as there is no water pressure.
What do I have to do to rectify this problem.