I would start by going into the Play Store
@cjprincess and searching for your Facebook Messenger App using the search car. When it appears, see if has an update notification next to it, and update the app if it does. Many app crashes are down to apps needing a current critical update to work properly.
Should this fail, I would long press the Messenger app logo in your main app selection pages. Hold it until a menu pops up on a grey tile. Now press the circled i symbol at the top right of the tile. Scroll down and select 'Storage'. On this page, go to the bottom right and press 'Clear Cache'. When your cache data returns to 0Kb you can leave by multiple return taps. I hope one of these solutions can work for you. Good luck.
