The live wire that makes all the lights on that circuit work may be open circuit - the wire snakes from one light fitting to another visiting each in turn - at the end it just stops and does not complete a loop like the power cables do.
Similarly the Neutral return wire - being open circuit - will show the same fault.
It's possible that you may have a defective circuit breaker on the downstairs light circuit but more likely a lose wire at a ceiling rose - somewhere near the consumer unit (fuse board) end.
If you're not confident then it's probably best to engage the services of a trained and certified specialist.
Remember Lord Finchley...
Lord Finchley tried to mend the Electric Light
Himself. It struck him dead: And serve him right!
It is the business of the wealthy man
To give employment to the artisan.
Peccavi, March 2013
