This is a very common requirement for many of us, and there are multiple ways to achieve this many of which requires running macros, but the easiest way that I follow is following these steps :
1) On the “Home” tab >Select the “Find and Select” dropdown menu ( on the Editing Group of icons)
2) Select “Go to Specials” and select the “Blanks” radio button and say “OK”
3) Type 0 or any character that you want to replace the blanks with
4) Press Ctrl+Enter
.. and you are done.