Why has one light stopped working when I disconnected one in another room?

Your problem would indicate that you have disconnected the feed to the next light in the circuit. Most ceiling roses are part of a house wiring circuit. They have three primary cables. Power in and out and a switch line. Power goes into a ceiling rose from a feed line. One cable goes out via the switch circuit to a light switch which turns a light on or off. Another cable goes on to feed the next ceiling rose in the circuit. This last cable has probably been disconnected when you disconnected the light.

Jeremy Travis
June 2007

lights run in series, power goes from one rose to another.
a bit like xmas tree lights only more power.
employ a competent electrician to restore power.

anon
June 2007