First step: restart the phone.
If that doesn't help, clear the app cache (Settings/Storage).
If that doesn't help, clear the system cache (restart into recovery mode and wipe the system cache only (don't do a system wipe).
If that still doesn't help, the phone probably has to go to a repair shop, because something came loose or broke when it fell down.
(Wiping cache doesn't delete any of your data.)