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.Android usually restores all settings after you re-install and log into the app. 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.Then, release the buttons and hold down 'Power' button until the screen turns on.Now you can try opening the app, it may work fine. Press and hold down the 'Home' and 'Power' buttons at the same time for upto 10 seconds. Try Hard reboot in your Android mobile.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 android operating system. Common Calibre Companion App Problems and Troubleshooting Steps ✅ I'm getting black screen / white screen (blank screen) when I open Calibre Companion?

Calibre app not working