These are the hated bugs - random. Very hard to repro and fix. Sigh.
An alternate to rebooting: Open Task Manager and kill the running Evernote processes. You can leave EvernoteClipper.exe - that's not what prevents EN from starting again. It's the Evernote.exe, EvernoteSubprocess.exe, and EvernoteTray.exe processes. When the main Evernote.exe crashed, it died in such a way that the other processes didn't catch that and proceed to act very badly.