Blank screen after pressing the phone icon

Hello everybody

When I press the phone icon in my iphone 5C I get a blank screen for 10 secondes and then the window closed to come back to the home page of the iphone. As a result I did not get access to the phone keyboard neither to the contact list nor the message box. I did a reset of the setting of my iphone but the problem remained. I pressed both the sleep / wake buton together with the home one but it did not work. I switch it off without better result. Anyone get any idea?

Thank you

Denis