I was working in Lightroom, tried to retrieve an image from a different folder, Lightroom got really really slow so I shut it down and tried to restart... it said something along the lines of "can't find Lightroom, do you want to find it or open a new one?" I tried to find it, couldn't, opened a new one - now all of the photos I was working on are gone. HELP!!
It must be the catalog file which Lightroom unable to find on the startup and you end up creating a new catalog, that why you don't see all the Images.
Open Lightroom while holding down the Alt/Opt key and it will show all the recently used or opened catalog on the startup and then you can choose to open the old catalog, else you find the Lightroom catalog to its default location.
Mac: /Users/[username]/Pictures/Lightroom/Lightroom Catalog.lrcat
Windows: C:\Users\[user name]\Pictures\Lightroom\Lightroom Catalog.lrcat