Quicktime player won't open

Boot into Safe Mode and see if it works as expected that way:

Use safe mode to isolate issues with your Mac - Apple Support

Kimajs Said:

“Quicktime player won't open on my computer. How should I troubleshoot this?“

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Use Command + Tab to Access QuickTime Player:

• Hold down the Command key and press the Tab key until QuickTime Player is selected.

• Then, see if QuickTime Player is accessible and usable.