Either there is an interruption (break) in the Live supplying the two switches or the Neutral return path from the lamps is broken. Any other explanation requires multiple faults to have happened at the same time.
You should find a way of testing for Live at the switches - if not there discover why not - if there examine the Neutral return path.
It may be useful to temorarily replace a dimmer with a conventional switch during your investigations.
Good luck... Take care...
Peccavi
February 2010