You've probably just opened the wrong (or a new) catalogue.
Search your computer for files called *.lrcat, and click on what you find. Clicking on them will open Lightroom with that catalogue loaded, and one of them, I'm willing to bet, is the one with the images you're looking for.
(Note that your pictures are never in Lightroom: Lightroom simply keeps a record of them, their location, and the edits you made to them, in its database. The images themselves will still be wherever you imported them to).