In most older houses all wiring was red, black and an earth wire with a green sleeve covering a bare wire. I suspect that when changing the light fitting you joined all the reds together and all the blacks together. The wiring going to the switch will be a red and a black that means the black wire when the switch is operated would become 'live' if that now 'live' black wire is joined to a black at the fitting it will blow a fuse and possibly some of the other wires apart, stopping other lights working. Also the switch contacts will have welded together making it necessary to replace the switch
AL snr
September 2009