Buy a screen from here: http://www.ownta.com/original-sony-ericsson-z520-cell-phone-lcd-screen.html
It might be also flex cable problem, you can buy from here: http://www.ownta.com/original-sony-ericsson-z520-cell-phone-flex-cable-with-ic.html
It's simple to repair it by yourself.
Lisain
August 2009