Around April 10th 2018 Facebook started making changes to application logins. Facebook is now requiring all their users to sign in again through the applications they use every 90 days and review the application permissions.
On April 17th 2018 Facebook started signing out users also from mobile applications if the login has expired. Facebook is showing all expired logins on the Facebook account settings - https://www.facebook.com/settings?tab=applications§ion=inactive
On Facebook mobile app these can be reviewed from the menu: Settings & Privacy > Account Settings > Apps > Logged in with Facebook > EXPIRED
To restore your progress please fully close and reopen the app and click on the Facebook icon on the top right corner to sign back in.