Power for lighting circuits usually daisy-chains from one ceiling rose to another - if you break the chain (Live and / or Neutral) all downstream lights cease working.
Your disconnected cable may hold the clue.
The last rose in the chain will have only two cables. One coming in from the previous rose and one off to and back from the switch.
Lighting circuits are often organised one for downstairs and one for upstairs - each with its own fuse or breaker.
Peccavi
April 2010