Had exactly the same problem (almost identical setup: Evernote 188.8.131.5255 (308555) Public (CE Build ce-62.3.7750), Dell XPS 13, model 9360). Evernote worked fine this afternoon, but suddenly this evening the left panel had disappeared and the "View/Left Panel" option didn't work. No obvious trigger for why it suddenly didn't work -- hadn't changed anything on machine, nothing had updated that I know of. Uninstalled and reinstalled and it worked. Annoying bug, now waiting for thousands of notes to re-sync.