Messenger is a server-based application, which means it requires a strong and consistent Internet connection to function properly. If your phone is experiencing network connectivity troubles, this is most likely the reason why the app isn't operating properly. The app may have become unreliable due to intermittent connectivity or Internet drops. In such a scenario, you'll have to address Internet issues first. Once your Internet is up and running, all online apps, including Messenger, should be up and running as well.

•  To do so, go to Settings->Wi-Fi and off the function by tapping the Wi-Fi switch. If the switch shows grey, it is off; otherwise, it is green (enabled).