To get help online, you need to provide as many details as you can. Otherwise, others will simply try to make some guesses. Anyway, If your files are damaged, you need to recheck the torrent and re-download them. But, If they are located on an external disk, and they are fine, maybe the letter of the drive where you have the files changed. In that case, from
Transfers tab, right click over the torrent, click on
Local files , then on
Location, then on
Move, and select the folder where the files are located. Or you can revert back the drive previous letter by following this tutorial: