Files marked by ! Exclamation Points are files that Lightroom has deemed missing from their expected location.
If you have Moved, Renamed, or Deleted a file or a folder outside of Lightroom this will happen. It can also happen on Windows Systems if your drive letter has been changed by the OS or on any system if the storage media upon which the originals reside is not connected or powered.
You will need to restore the location of or repoint to these original images.
Downloading Lightroom and reinstalling will not fix this problem.
The exclamation point means that Lightroom cannot find your photos, probably because you moved them (or renamed them or deleted them) outside of Lightroom. (Hint: don't do that any more)
To fix the problem, see these instructions: Adobe Lightroom - Find moved or missing files and folders