I, too just made this fat-fingered mistake, removed a tag from a ton of research notes, and because I had searched on the tag which was just deleted they immediately disappeared from view, so I could not re-tag them with it, or even find them all again.
I don't have a second computer with Evernote on it, and, it synced immediately to the web, destroying any hope of disconnecting and salvaging years of work via the website.
I have, however, found a 100% effective solution going forward, which doesn't require me to interrupt my work to remember to make frequent manual backups:
Switch to an online-synced note taking app that takes users' work needs seriously and doesn't leave inexcusable, work-destroying bugs like this in its product unfixed even as users complain over and over about this same issue for over 7 years. Presto! Problem solved. Goodbye, Evernote, you've finally lost me for good.
This is inexcusable. There should be NO for way a single wrong keyclick to permanently destroy years of organizational work, without any 'undo' feature, especially the better part of a decade after users first complained about it. At least, not in a professional note-taking app.