Hi
It may help to close the Messages app and then to restart your iPhone (normally):
- To close the Messages app:
- Double click the Home button > swipe left / right to the Messages app > swipe up on the app preview to close it.
- Force an app to close on your iPhone, iPad, or iPod touch - Apple Support
- To restart your iPhone, follow the instructions here:
- Restart your iPhone, iPad, or iPod touch - Apple Support
- (Note that Apple advises that you should not force restart your iPhone unless it is unresponsive and cannot be restarted normally).
If you have not yet done so, it may also help to update your iPhone to the latest version of iOS, being sure to make a backup first (if using iTunes, you may wish to select the option to encrypt your backup):
Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support