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Go to Settings -> About Phone -> Diagnostics -> Test -> Touchscreen, and verify that all parts of your screen can be activated.
Do you have a phone case installed?
You'll find a link on my previous post, if you can't find it go here:
/t5/FAQ/How-to-boot-your-phone-into-safe-mode/m-p/348008#U348008
If the top of the screen isn't working at all then this sounds like a hardware issue not an app or apps issue so you would need to either return your device to the place of purchase or contact Xperia care and have your device inspected http://www.sonymobile.com/global-en/support/contact-us/