You need to relink them.
Right click on clip and relink media.
I assume by saying "lost" you mean they are not linked to the project anymore but still on your hard drive and you know where.
What you need to do is relink your media.
Normaly in your case if you open the project, a window should pop up asking where the files are, and you just pick the right directory. In most cases you only need to prompt to one file and premiere will find the others by itself.
But you can also do this after you already opened the project:
Rightclick the "lost" clip in the bin and select "Link Media". The same window will pop up, asking for the place on your harddrive to find the asset.
You can select multible clips and relink them at once if they are in the same folder, but you need to repeat the steps if they are spread at different locations on your harddrive.
So closing and reopening the project to relink all the media sometimes is faster.
Thank you for the tip. I was able to re-link the files. In the future I will have to be more careful where I import my files.