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.
.mov is not a audio/video format, it is a wrapper format. There are any number of video and audio formats that are supported by the .mov wrapper. You need a compatible codec for each. I note that the OP did not give the camera manufacturer or model. This missing information is important because some manufacturers use proprietary-format video which requires a proprietary codec. In this case, the required codec is included with the manufacturer's software in the shipping box.
Many manufacturers use video formats that are supported by the Perian codec suite. If the OP has not installed it, then I strongly recommend that he install Perian.