RedHenry
Do you mean, the app doesn't exist anymore on your device or do you have an issue to login?
If it is the first, then maybe you have deactivated the ProtonMail app accidentally?
Then it would also disappear from the home screen…
The easiest solution would be to download the App Store app that was used to install the Proton app and then request "Settings". The app info will then be visible again, where you can tap "Activate".
This is how it works with Neo Store. And maybe the same with your app source, whatever it is.
Otherwise, only reinstalling will help.