I lost one of my iPhoto libraries

Lost in that it has been deleted - or lost in that you just don't know where it is.

and iPhotos, or Photos.

Either way use the spotlight search (search in finder). Put photolibrary in the search box for iPhoto, or photoslibrary for Photos.

(if you are not sure, search for both)

Make sure "this mac" is selected.

If you click on the found libraries in turn, the location will be shown at the bottom of the finder window.

Thank you - it has been restored.