Lr stores - among all the other things - the file names and file locations in the catalog and creates a link between the catalog and the image file.
When image file names or their location is changed outside of Lr - or outside of a catalog - this link is broken and you get the "missing file" message to make you aware of the broken link.
In your case, you opened an older catalog. But you probably have changed image files and /or their location since you made the backup that you now loaded as catalog.
So the situation in the catalog doesn't reflect the situation on the hard drive and you get the "missing file" message.
The remedy is to re-link the image files / folders. The remedy is not to re-import because you would loose your edits by re-importing.
See here on how to do the re-linking:
http://tv.adobe.com/watch/creative-suite-podcast-photographers/lightro om-dude-where-are-my-photos/
http://www.computer-darkroom.com/lr2_find_folder/find-folder.htm