Combi drain down now no hot water !?

I have a five year old Biasi boiler. It has been working fine. Today had to swap radiator and drained down system ready for more work tomorrow. Set boiler to Direct hot water only to run a bath and the boiler fan runs when the hot tap is turned but boiler does not fire. After a few seconds the warning light starts to flash 4 flashes per second and the boiler turns itself off. It doesn't lock out by the way just fails to fire. Have not done anything else to boiler and can't find manuals !!! Please help - need hot water !