It is always good practice to restart your phone, at least every couple of days, and to close down Apps when they are no longer required.

Restart your phone, then restart the app.  If that doesn't work, try forcing the app to close and clearing its cache.

1. Open the ‘Settings’ app.
2. Click ‘Apps and Notifications’.
3. Click the name of the app. You may have to click ‘See all <X> apps’ and scroll through the list to locate it.
4. Click ‘Force Stop’.
5. Click ‘OK’ to accept the warning.
6. Click the arrow in the top left hand corner of your phone to go back one screen.
7. Click ‘Storage and Cache’.
8. Click ‘Clear Cache’.
9. Click ‘Clear Storage’.
10. Click ‘OK’ if you are warned about deleting the app data.