If you have been using your phone to access Facebook then it means that your app version of Facebook needs to be updated.Its always behaving this way especially when its not updated. Otherwise you can try installing Facebook lite only if you've been using your phone's Facebook.
Also, this issue might have risen after your cleared the Facebook data cache.All you have to do is uninstall the app then install it again and it will surely work.Also you need to check your internet connection.
Finally, another alternative, you can use the web to login in to your Facebook. This must work no matter what.Just try it out especially try logging in with the chrome.