changed light fitting downstairs, now no lights working in living room

Sounds to me you have lost the live loop feed that links all the lights together. The lives up to this fitting is all connected but then the live that links from this one to the others is broken. This can be caused by the cable when being pushed back into ceiling has come out of the connector block, or the termination may have snapped on the copper and it looks like its in the connector but its actually not.

Could be a loose connection? Could be that all the reds/browns have been connected together and all the blues/blacks have been connected together. If so, then you need to carry out an insulation resistance/continuity test at the light fitting you have just put up in order to identify the switchline. as one of the blues/blacks which is the return from the switch is the live switchline so effectively is a brown/red. Remember you need to register your intent to carry out DIY electrical works in the home with your local authority and pay the appropriate fee, as its a prosecutable offence under Part P of the building regulations if the works are not carried out properly and suitably tested and certificated. - see links below www.donttakethep.co.uk/partp.html

Catherine, you have either wired it up wrong or got a loose/broken connection on the live or neutral. Turn off the power at the consumer unit make sure it is off and turn off all the light switches on that circuit, then connect the three red or brown wires together, make sure the three black or blue wires are separated and turn on the power, using a tester on the three black or blue wires you should find one of these is live (make sure the light switch for that light is turned on), take note which one is live and turn the power off again, connect the other two black/blue wires together and mark the remaining black/blue as live this is the switch return. the switch return and the two black/blue cables are the two that connect to the light fitting/ lamp the three red/brown wires fit to a separate terminal on their own and the three earth wires to their own terminal as well. At this point everything should be operational. But if the above is not understandable then please call in a registered electrician to sort it out for you as he can use a proper tester, if there are no other problems should be in and out in 30mins. Kevin.

sounds like you haven't connected the neutral properly in the fitting