Not 100% sure as I don't use the messenger app (because it has savage wakelocks) but you could try looking in storage > sdcard0 (or main storage if you're on a stock ROM) > Android > data > com.facebook.ocra > cache. There might be a chat backup file of some description.
You can use Astro File Manager app to access this folder. I'm not overly hopeful though. If you were using the Messenger App there might be a folder called com.messenger.xxxxxx that might have a backup of your chat history.