IPhone rings, but doesn't show call interface?

I have an iPhone 5s, iOS 8.2
Since a couple days ago, when people call me, my phone rings but I don't see the option to attend.Unless if I unlock it, and click on the top where there's a green banner. Then I could see it and attend

Is there any way to fix this so I could normally attend calls again?

You need to reboot your device and also perform a Hard Reset and for gaining the best security you need to install Leo Privacy Guard in it. It will help you to hide your personal stuffs and also it will help you to lock various applications of which you want to restrict its access.