According to The Verge, that’s because you downloaded Messenger version 170.0, which was just released yesterday. It seems to have a bug which makes the app crash as soon as you open it, according to many user reports.
The fix is just as simple: Download Messenger version 170.1 right now. It’s brand-new, released today, presumably just to counter that bug. Facebook confirmed to CNET that the new update should solve the issue.
Sure, you could maybe just wait for it to update automatically, but perhaps you’re away from Wi-Fi right now. Depends on your settings, I suppose.
Disclosure: Sean’s wife works for Facebook as an internal video producer.