It may be too late to get useful information out of this question since you've started troubleshooting already, but I'll ask anyway:
Did you change (or allow to be changed) anything on your system just before Pr suddenly stopped working? If so, undoing that change may put things right again.
Regardless, try this:
Launch Pr from a shortcut (taskbar, start menu, Desktop) while holding down the Shift+Alt keys. Alt resets prefs and Shift clears plugin cache.
Keep the keys pressed until the main program window appears.
Before trying to open any project or starting a new one, go to Preferences>Media Cache and delete *all* your media cache files.
Now try opening a project or starting a new one.
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