From this site, Olympic Flame - Wikipedia, the free encyclopedia :
Re-igniting the Flame
It is not uncommon for the Olympic flame to be accidentally or deliberately extinguished during the course of the relay, and on at least one occasion the cauldron itself has gone out during the Games. To guard against this eventuality, multiple copies of the flame are transported with the relay or maintained in backup locations. When a torch goes out, it is re-lit (or another torch is lit) from one of the backup sources. Thus, the fires contained in the torches and Olympic cauldrons all trace a common lineage back to the same Olympia lighting ceremony.
One of the more memorable extinguishings occurred at the 1976 Summer Olympics held in Montreal, Canada. After a rainstorm doused the Olympic flame a few days after the games had opened, an official re-lit the flame using his cigarette lighter. Organizers quickly doused it again and relit it using a backup of the original flame.
At the 2004 Summer Olympics, when the Olympic flame came to the Panathinaiko Stadium to start the global torch relay, the night was very windy and the torch, lit by the Athens 2004 Organizing Committee Gianna Angelopoulos-Daskalaki, blew out due to the wind, but was re-lit from the backup flame taken from the original ceremonial flame at Olympia.
In 2008 the Olympic torch was extinguished at least two times by Chinese officials (five times according to French police) so that it could be transported in a bus amid protests while it was being paraded through Paris. This eventually led to the cancellation of the relay's last leg in the city. The flame itself, however, remained preserved in the back-up lantern used to keep it overnight and on airplanes, and the torch was relighted using this.

