wp-admin shows blank page, how to fix it?

I am working with WordPress 3.5.8. Yesterday I made some changes into it to a friend. I don't know what happened, but after some time when I try to log in at wp-admin side and execute URL www.mysite.com/wp-admin it shows me a blank page. How do I solve this? I have searched over the internet and blogs. I found it could be related to hosting problems or brute force-attack, but I don't think that that's what happened to my site.

Links I visited:

• http://wordpress.org/support/topic/wp-admin-wont-load-shows-blank-page
• http://wordpress.org/support/topic/brute-force-attacks-and-wordpress?replies=1
• http://wordpress.org/support/topic/wp-admin-wont-load-shows-blank-page
• http://wordpress.org/support/topic/wp-admin-blank-page
• http://wordpress.org/support/topic/blank-page-on-wp-admin

Try turning on WP Debug. If this is happening due to a PHP error (which I bet that it is), you will be able to see what's going on and fix the error.