Can't find file in Lightroom 4

[batmura]

Hello!

I hope someone can help me with this problem. I can't find a certain file that used to be on Lightroom 4 such that when I turned L4 on I would see it among the pictures at the bottom. Today, I imported a new set of photos into L4 and whenever I do that the previous photos that I used to see at the bottom are replaced with the newly exported shots. When I go back to my external hard drive, the photo is dimmed but I don't know how to find it in L4. It no longer is in the folder I thought it would be. I tried the Find file option in the program to no avail. Since it's dimmed in the external hard drive, does that mean the file is imported or does it mean it's no longer accessible?

Thanks!