If you go to the
Previous calibre releases page, you can find previous calibre releases there. You can try to download and re-install the .msi file for calibre 7.21.0 and then try installing 7.26.0 over it.
On a different topic, I would never let Revo Uninstaller near a computer I was using. Several people I knew who used it ran into issues where files that were used by other programs were removed and I've had fun repairing the damage.