When a SharePoint List/Library is marked hidden, SharePoint simply removes there navigation point from the SiteContents. The actual List/Library still continue to exist at their respective urls, the format of which remains in sync with any other SharePoint List/Library.

So to access a hidden SharePoint List, navigate to:

[siteurl]/Lists/[listname]/AllItems.aspx

For document library, use the following:

[siteurl]/[listname]/Forms/AllItems.aspx

Key Takeaways

• This is assuming that your SharePoint List/Library still have the view, AllItems.aspx. If not, then replace this name from the url with your custom view name.
• If your List/Library name contain space characters, say My Custom List, then do replace all the spaces with, %20. So your url in this particular case will become:
  

  [siteurl]/Lists/My%20Custom%20List/AllItems.aspx