how to change master page (apply old 1 as new 1 showing blank view)

Hi,

I have designed and apply master page on my portal,its showing blank (nothing available).

I want to apply my old master page .

Problem:

as I type my "url "of portal, its showing nothing even if I type like layouts/15/changemasterpage.aspx 

even this one is not working.

Solution Required:

just let me know how can I apply my previous master page back so things working fine previously to me