I have been working my way through my house, replacing light switches and fittings, all with no problem. I have relined the walls on the last room. Before doing so, I connected a junction box at the position of the original ceiling rose. I then connected a temporary light to one of the new outlets I created and connected the old light switch. Everything worked perfectly. On completion of the lining, I changed the old light switch for a new one. Now the entire circuit is dead. I have tested the voltage but the only reading I am getting is 12v. Nothing has been moved and nothing in the new wiring layout has been disturbed. I am totally puzzled. The only thing left to do seems to be to strip out the wiring and completely rewire, which means a lot of work taking out recently installed lining which I am naturally unwilling to do. Any suggestions?