Phone won't shut off

My wife just got a Lg optimus extreme.She is on net10.Sometimes the screen

goes black and she can't get it back on.The phone stays on but you can't turn

it off if you're talking to someone.You can't get to dialpad etc.She needs help

via Forums de téléphone cellulaire (en français) http://cellphoneforums.net/fr/lg/t364200-phone-won-t-shut-off.html