From the grid in the Library module, right-click on the missing picture, select "show in explorer/finder".
If the file is not on the hard drive in that location, you really deleted it.
Check your "recycle bin" ("trash"?) to see if you can undelete it.
Your pictures exist on the hard drive like all other files, not "in" the LR catalog. The catalog just references the pictures by their path and file name.
need help retreiveing a file i deleted.The photo is still their but says "the file could not be found"
Does the photo have an exclamation point icon (as shown in Figure 2 at Adobe Lightroom - Find moved or missing files and folders) or a question mark icon?
If so, then follow the instructions at that link to reconnect the photo.