Today, a friend called me on my land line to ask why I am not answering on my cell phone. I grabbed my Xperia S and found out it was vibrating and the strip line was lit but the screen was black. Nothing I did (from touching the screen or pressing any mechanical button) could open the phone. So I had to force it close with the power and volume button and restart it again. The only thing I did lately was upgrading to 6.1.A.2.55. By the way, the wifi and 3G were disabled when this happened and I use the vibrator only when the phone is in silent mode which was not the case this time. And the call from my friend was not saved that he called. Any Ideas why?