My built in ads extension stopped working after updating unity? How can I fix this?

Well the title says it all, after updating Unity the built in ads extension for unity stopped working for me. It now says that using UnityEngine.Advertisements doesent work and the namespace Advertisements doesent exist. What could be casuing this problem?

this is what it says in the error message:
FileNotFoundException: Could not load file or assembly ‘UnityEngine.Advertisements, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null’ or one of its dependencies.

Have you checked in the Package Manager (Window > Package Manager ) maybe you need to update the built in ads extension there? Or maybe it not even enabled for some reason.