Few years ago I got an American Visa in order to work there, but at the end I did not travel, this year I want to apply for a Schengen visa. Could I have any problem since in my passport I have an American visa that has expired?
An expired visa in your passport should not be a problem. Perhaps they may wish to know why you didn't use it, but if anything having been approved by the USA is a good thing, as USA visas are often more difficult to get than Schengen visas.
darn, Relaxed types faster than me ;-)