If you still have the originals (e.g. on the card) you could try to import them again.
VLC is sometimes able to repair a broken video file. When you open it, VLC will say it's broken and will ask you if you want to repair it. If VLC doesn't ask this, it can't repair the files.
I'm not familiair with AVCHD, but it could be that it shows up as a .mov file (I could be totally wrong here). With AVCHD it can be important to copy the entire card (so make a digital image of the card). The hierarchy and structure of the files is important for quicktime to play the video back.