This happened to me for the first time today on Win 7 Pro 64. I used Process Explorer v16.05 (https://docs.microsoft.com/en-us/sysinternals/) to kill all running instances of Evernote, (there were four, including the tray application), and then Evernote opened normally after that. This is a common occurrence with other Windows programs; it's surprising how often I need to do this. It suggests also that shutting down and restarting would have also fixed the issue, but I can't be sure.
Hope that helps!