Are there any examples of technologies have been lost over time?
At least four examples spring to mind: Damascus steel, which might have been rediscovered last century, Greek fire, whose composition is still a matter of debate, Roman concrete, whose formula was lost in Western Europe after the fall of Rome and later rediscovered during the Renaissance, and the Antikythera mechanism, an ancient Greek clockwork device that was discovered early last century and only recently established to be an astronomical clock.
There's also the stuff of legends, whose accounts attract the skepticism of modern scholars (like burning glass) or of contemporaries (like the circumnavigation of Africa by Phoenicians).
If you feel like sitting through an hour and a half long video, this example of traditional African iron smelting was nearly lost -- it would have been had it not been for a desire to preserve it. The video will walk you through the whole thing: creating the charcoal, creating the furnace, and then operating it.
The latter video is, I think, interesting in that it hints at how much technical knowledge may have been lost over time without us necessarily realizing -- at times while leaving a trace that can later be rediscovered, and at times not. Iron smelting technology is alive and well, but had it not been for conservation efforts, the above video would never have been created, and how Africans smelted iron in that area would have been lost to time unless later researchers would have succeeded at recreating it through detective work.
In this sense, obsolete technology is comparable to a dying language. At one point there are only a few people left who know it. And then comes a day when the last person who knows it passes away. But their reason for existing stays with us in some form or shape and they simply get replaced.
As a last illustration of the above, consider the (actually Celtic) Roman saddle, which all but disappeared after the introduction of the stirrup:
How they worked was reconstructed last century by Peter Connolly.