QuickTime doesn’t support all file types. For example, MKV or MTS files won’t open in QuickTime as it doesn’t support these formats. Hence, if the video player doesn’t open videos on Mac, try converting the video file format to MOV or MP4 using a converter tool or play in another media player. If these don’t work, the video file might have been corrupted. Repair it using a Mac video repair software shared in this post.