Why I cannot access Lost password/Register/Login page on my site?

First of all, you need to go to Appearance > Customize > Content restriction > All pages, and make sure the restriction is not enabled as you hide all pages. 

Surely, you can use this option but you need to specify the "Exclude pages". 

We recommend you Exclude the following pages: REGISTER, LOGIN, FORGOT PASSWORD, ACTIVATE, Terms and Privacy policy.

More detailed information about the  Content restriction you can find following this link.