Navigation sidebar disappeared?!

Hi, I’m a new user, I had a list of basic pages and articles on the standard sidebar navigation but now it has disappeared and can’t work out how to get it back. All the content is still there.

It started when I tried to crate a header block to try and place an alert on the homepage. This resulted in the all the navigation links from the articles created disappearing from the left hand side. How do I get these back? or do I have to create a whole new sidebar block and create these articles again?

Any help appreciated, first time in using this website and don’t understand how to configure the blocks correctly.

Drupal version: