It sounds like you may need help reinstalling Pandora on your iPhone.
You can find our app in the App Store on your device. Search for Pandora and tap Install.
You can also go to pandora.com on your phone or tablet's web browser, scroll down and select Get the App to install the app.
Downloading any app from the App Store will require that you enter the password for your Apple ID. Keep in mind that the email address and password for your Apple ID account may be different than your Pandora account.
Also, please be aware that Pandora requires iOS 12 or higher. If you are running iOS 11 or lower, you will need to update your iOS version.
If you're not sure how to update the OS on your phone, you can refer to this article for more help.
Let me know how it goes.