AdMob banner ad not showing

Question

I am using admob in my app, it was working fine but suddenly it stopped showing ads at all. Bellow is my codes i used:

mainActivity.xml:

Answer 1

Did you go back to your admob account to register your specific app for ads and get a new longer publisher number with the 'ca-app-pub-' preface instead of the 'pub-' preface?

Nowhere in the "banner ads 1" instructions on the Google Admob "Google Mobile Ads SDK" development site does it mention having to go back to your admob account to do this.

This stupid mistake held me up for days.