Hey, at least it's doable. Just open the file manager in the 7zip folder start menu thing, then open the file you want, or unzip it, whatever. It isn't the end of the world, I promise.
The context menu integration is still available. Open 7-Zip file manager, goto Extras menu and select Options. Go to the 7-zip tab and check if context menu options are selected. Tested with latest available version 18.05. If this doesn't work, try to start 7-zip in administrator mode and repeat all steps.
Steffen
The man with the big fett Bauch, caught in the back of a Knallerbsenstrauch!