Apparently QuickTime Player .mov files that could be opened by QuickTime Player before the installation of Mavericks OS can no longer be opened by the new version of QuickTime Player which is installed by Mavericks and later. Since Mavericks has been installed, I have not been able to open QuickTime Player .mov files with the version of QuickTime Player installed by Mavericks and later but I have been able to open the QuickTime files with QuickTime Player 7 which still works under Mavericks and later. At this writing, I am using El Capitan OS and QuickTime player 7 still works and I am able to open the pre-Mavericks QuickTime Player 7 .mov files. The catch is that even though I can play my old .mov files in QuickTime Player 7, I am not able to convert them in QuickTime Player 7. I am only able to open the same .mov files in QuickTime Player 10 which is the version I am using now on El Capitan after a forced conversion to QuickTime Player 10 .mov format which takes a long time and makes the file much larger than is desirable.
Here is an Apple forum discussion that covers some more detail that I may not have covered on this issue. This is a very frustrating situation as I have files that are only in .mov and the only solution is to convert them to the latest version of .mov and accept the new bloated file size.