Need Help Urgently - First Floor Lights Have Gone off

[ineedhelp]

Hi
I live in a new build house (10 years old) and suddenly the first floor lights went off. No lights were switched on at that particular moment although three room lights were on at the time.
I checked the main fusebox and only the first floor lights switch in the mains had tripped off. I put off all the lights in the first floor, removed the bulbs and switched it on at the mains but still the lights won't come on. The appliances through the plugs are working fine in the first floor and so are the entire ground floor.

Can you suggest what I am doing wrong or if there's a simple solution?
Thanks in advance
Simon

[ericmark]

MCB's don't often fail but normally you can feel a difference with the reset leaver when they do.