No, not a new user - I've been a premium user since 2011. The punch line is that a complete reinstall today fixed the problem, but I still don't know what happened in the first place. To answer the questions you ask, though, which I should have done from the start: I did update to v10 a few weeks ago, really disliked it, so uninstalled pretty promptly. Went back to 6.25, but it was that "legacy" version (6.25.2) that kept encouraging me to upgrade, so I uninstalled THAT and used my original 6.25.1 installation file, which was still sitting in my downloads folder. That was working just fine for a few weeks until this weekend, when it just ... stopped. Would not even open, just pretty quickly went to the Windows "not responding" message and I was stuck. I tried uninstalls/reinstalls, with no change, so today I manually deleted *everything) Evernote-related on my hard drive, the entire folder and subfolders, then did another uninstall of 6.25.1, and then another reinstall and so far, so good.
No earthly idea why it crashed, but happy to have 6.25.1 back and (fingers crossed) working as normal. Appears that getting rid of the database plus all related files and forcing a full re-download may have been the key?