I'm trying to find a sci-fi book (or possibly series) that I read in either the late 1990s or early 2000s. I think it was on Earth, but most civilisation was gone (rediscovered Eden type sci-fi). The protagonists find bits of old technology like mobile phones (possibly they called them pearls?) that don't work very well. They might have been nomadic tribes and there weren't any cities left?.
Another answer, if you are talking about a series, would be Crusade, also Babylon 5 to an extent. Crusade follows a ship of people looking for old or lost technology that might bring a cure to a plague that has infected the earth and will cause all life to end in five years.
Crusade was a spinoff of Babylon 5 which also dealt with races thousands of years old possessing technology far beyond ours. A number of episodes dealt with finding ancient technology, for example episode 4, Infection, tells of Ikarra 7, now a dead world, was a thousand years ago home to a highly advanced space-faring society. Their technology was organic: tools and artifacts made of living tissue yet immune to decay. Invaded over a dozen times, they finally built 12 devastating organic warriors to protect them. Programmed to destroy any but "pure Ikarrans", those warriors repelled the last invasion and went on to kill any Ikarran who deviated from the ideal (ie. all of them).
Another episode, In The Shadow of Z'ha'dum, starts telling of the old races and their unimaginable technology.