The loop provides a live feed to all the lights on that circuit, probably for the whole floor.
At the rose you would take the live down to a switch, then back to the outer terminal of the rose and on to the fitting.
The wire used for the switched live will be black or blue with an identifying sleeve (or bit of tape) around it. Remember it is a live and not a neutral.
If you've wired it the same as it was before was it not working before?
Adam
August 2009