logic combi 30 shows f3 fault?

been working ok all winter until yesterday when i switched the whole boiler off.
i'd only done this because i reckoned we were about to run out of gas as we are on a PAYG meter.
I reasoned that like a car it would be more difficult to restart if i actually let the boiler run out of gas
now i've topped the gas up the boiler wont re start and just shows fault F3
is there anything i can do? i can't believe there's anything really seriously wrong
thanks