If you moved the folder or location containing your actual video and sound clips, you must move it back to the exact same location. Unless you specified during the import process that you wanted to copy your media to the event you selected, FCP X will use aliases in the (Event Name)/Original Media folder. These aliases will be hard coded to point to the original location of the file and the original name of the file. I have learned, at least in my experiences, the red icons will not always disappear when you get the files back into the original location. However, if I did another Import (File) of the "missing" file into the same event that contained the original clip icon, it would fill in the red icon with the clip video and, if that clip had been used in my timeline, it would propagate the "refresh" of the media down to the timeline and everything would once again be happy.
If, in the future, you are looking for a way to put your files on another drive, check out the following video ...