You can also try restarting Mac or re-installing QuickTime to fix the QuickTime not playing MP4 or MOV issue.

Otherwise, you can rely on a media player like VLC to help you view the video. You need to rely on a video repairing tool if it is a corrupted file. You can try playing the media file with another player to check if the file is corrupted. You should know that, many unsupported video codecs will cause the QuickTime not playing issue on Mac. When you encounter the problem that QuickTime MOV not playing on Mac, or QuickTime Player can't play an MP4/AVI file, you should first play the file with another media player. How to Fix QuickTime Player Can't Open MOV/MP4 File You can uninstall the QuickTime Player and then back to the App Store or official download page to install the latest version of QuickTime Player to your Mac. To fix the QuickTime not working on Mac, you can also choose to re-install the QuickTime software. After that, you can try opening QuickTime Player again to check if the problem is solved.

Wait for a few seconds and then restart Mac.

You can take the regular way to shut down your Mac. When you get the QuickTime Player won't open issue, you can try restarting your Mac first to solve the problem. In that case, when you run the program, the video playing window won't appear on the desktop. Various reasons will cause the QuickTime Player won't launch problem, such as a corruption in the macOS, or in the files associated with the QuickTime. How to Fix QuickTime Screen Recording Errors

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