Archived goals behave a little differently that other goals.
They won't show up in your searches by default and the goal is essentially 'locked for monthly updates'. This means that you can't add monthly updates to it anymore.
However, you can still
The main purpose of archiving goals is to allow you to clean up those goals that are perhaps old and have been completed. While still retaining them somewhere should you need to refer to it in future.
Learnings from archived goals will still be visible in other goals that are tagged with the same tags. And as such, they will show up in topics.
You can restore the goal by clicking on the link in the banner:
Or using the restore button in the right hand sidebar: