No permissions to access site root "Sorry, you don't have access to this page"

If you have had this annoying message on your site

Sorry, you don't have access to this page

and you don't understand why it's happening, try the method below:

You have given read permission to a user on Site Permission and even with full control permission, user still can't access the Site!! keep reading...

You have tryied out every internet solution and nothing was usefull ?
Wait, here is the solution.

1- First go to your root site and list all webparts on it with "?contents=1" at the end of URL.

2- After that check every webpart/list/library permission that says "open on page" to see if it has "unique permission".

3- If you find any webpart/list/library with "unique permission" then "delete unique permission" and you should now login fine with the problematic user ;)