It seems that you have done some changes to your website and at some point, all pages except the main one started showing a 404 error. 

No worries though, it’s very easy to fix, let me show you in details how to do this 😊 

To fix the error all you have to do is to rebuild your permalinks. Please open your WordPress admin panel (domain.com/wp-admin), find menu item Settings and click on Permalinks: 

Before doing any changes please remember what type of permalinks you had, save this information separately if needed.

After that, it will be enough to select any of the option that does not match the current one, save the changes: 

And then again set the option you need.

Done - permalinks have been re-created, the site should work correctly 😊 To see the changes, refresh the page via Ctrl + F5 or open it from the incognito mode.