This happened to me once, I'm afraid it was something needing repaired (I can't remember what though), call a central heating engineer (with no call out charge) and advise you.
Please don't mess about with your boiler yourself!
Maybe the pressure has dropped or it could be a broken fan. I think it's best if you get a Corgi registered engineer to come and have a look.
I have British Gas 3 star cover for my boiler. I'm so glad I got it, as in the first 3 months, my boiler broke down 4 times. Had it not been for my cover, I would have spent a fortune calling out engineers.
It is a legal requirement that anyone installing or maintaining a gas appliance is CORGI registered, although many who are not (for example those whose registration has lapsed) know what to do. Almost all those who are (or have been) registered are technicians, not engineers although the term "engineer" is widely used inaccurately as any qualified engineer will tell you.