about Samsung Galaxy s4 mobile phone android 4.2.2

My Galaxy s4 LET phone does not working any more. I have bought phone before a month and the phone take sms allow to use mobile internet and etc but it does not ply any video like on youtube and others...and does not gives any sound sound is totally dead. The phone was ok before. one day as soon i disconnect the phone after calling and again i tried to ring him after 10 minutes i had noticed that the phone went to black screen after i hit on call buttom on scree. Anyone does have any specific idea to get my phone back to normal.
thanks