If that is your case, try installing older versions of the app. Even in some rare cases, the re-install step also don't work.iOS usually provides an option to restore settings and content from iCloud backup, if you've enabled it previously. Finally, if you can't fix it with anything, you may need to uninstall the app and re-install it.After that put it to charge, and press the power button. If none of the above working, you can wait till your phone battery drains and it turns off automatically.Press the power off button for few seconds and then choose restart from the menu to restart your device.Now you can try opening the app, it may work fine. Then you close the app that has this issue. You just need to press the recent applications menu (usually the first left button) in your phone. Most of the times, it might be a temporary loading issue.Usually when you open an app, you will see a black screen for few seconds and then app will crash with or without an error message. It is one of the most common problem in mobile phones and tablets. Common Calibre Companion App Problems and Troubleshooting Steps ✅ I'm getting black screen / white screen (blank screen) when I open Calibre Companion?

Calibre companion app