Failed Circuit Breaker?

[Please say which country when posting to an international newsgroup. I assume UK (my answer would be different for some other countries).]
It's very rare, but not unheard of. (BTW, closed is on for electrical switches/breakers.) Did you try switching it off and on again? More likely is the conductor is broken or come out of a terminal in the wiring. Has anyone done any electrical or other building work, even just drilling a whole or hammering in a nail recently? Have you tried _all_ the lights? What was going on when it failed?

Thanks for your reply. Yes I am in the UK. Nothing unusual was happening when we lost the downstairs lights. In fact it was during the day, so we only noticed when my wife tried to turn on the kitchen lights. The ciruit breaker had tripped (sorry made mistake earlier) reset it, but the lights did not come back on. Tried resetting it again with no effect. Have tried all the downstairs lights which are on the same circuit and none of them work. After my last post I took the front off the fuse box and checked that the connections on the circuit breaker were tight. Still no joy. I guess the next step is to replace the circuit breaker?

Ah OK, that is perhaps a more common failure mode. It could be that the breaker itself broke. One reason could be that it couldn't handle the breaking current in the case of a short circuit. What make/model and exactly ratings are written on it? Another cause of breakers not switching back on is getting plaster/brick/masonary dust in the breaker; you need to keep that out of fuseboxes.
I would probably temporarily connect the circuit to a different breaker of the same rating and see if it works. Of course, if there is a short on the circuit and the breaking capacity of the breakers is too low, you'll probably lose another one (possibly explosively), but you'll need to replace them all in this case anyway as they aren't safe.

How old is the breaker? IMO, if it's over 40 years old, then it's questionable and should be replaced.

Thanks everybody for your helpful advice. I replaced the circuit breaker this evening and it resolved the problem. Thank you all very much, its nice to know there are helpful people out there in this sometimes very cynical world.