OK, I assme that you changed the file names outside of Lightroom, then -- not a good idea! The Lightroom catalog stores file info in order to keep things organized and keep needed metadata (including editing/processing info) and, as you've seen, trying to do "stuff" outside of LR can mess things up, as in "losing" its whole connection with the files!
I'm not sure exactly how to "fix" this -- you can try to change the file names back to the original names using your regular sytem naming (in other words un-do what you did) and then see if LR can re-recognize them. Before doing anything, I'd back up the images as well as do a Lightroom backup so you will have a point to return to if all else fails.
If the renaming thing doesn't work automaticall, try selecting the folder and doing a Synchronize and see if that works.
If that doesn't work, well, you may be stuck with removing the "ghost" images from your LR catalog and then re-importing from scratch, but of course you lose all metadata/editing info...
And then, in the future, remember that LR is designed for image organization, including renaming files while keeping them "in-house" (you can do this by right-clicking or using a menu function). Of course the Export function gives you control of renaming exported files (not ones in your catalog, unless you specifically re-import them).