If nothing works, you can do two things:
a) download the old installer. You will find it here:
https://download.calibre-ebook.com/ and install it again.
If this isn't working for you too, you can do it on the hard way.
b) Delete the program directory (see
https://manual.calibre-ebook.com/faq...is-not-working). This will not delete the library or your calibre settings. Both have different locations and calibre will find them with the new new installation.