If combo boiler heater check the gauge on the dashboard it must read 1.5-2.0, underneath the heater/boiler there will be a thin copper pipe with a tap at each end, open both taps inline with the pipes and watch the gauge till it reaches at least 1.5 then turn them both off, your machine should then work,
It is a safety measure to stop the pipes being burnt out through lack of water in the system.
Rick
August 2016