Lighting not working?

I installed a new light fitting which worked for two days. I switched the light on on the third day and it would not work. I assumed it was the light but then my kitchen light stopped working. I took the kitchen light down and fitted it in the bedroom. It worked but when I refitted the original bedroom light this didn't work. Now all upstairs lighting has gone except for one bedroom. Also all lighting downstairs has gone except for lounge and hallway. All wires remain live even when light is switched off. Any ideas what problem is?