P2415Q doesn't turn on connecting with display port cable

hello

i have computer with Sapphire hd 7970 , it has display port and HDMI outputs.

i connected the cable i received with the monitor DP to mini DP and it worked ok for a week,

today i switched on my computer the computer turns on i can hear the windows starting but there is 

no signal on the display,so i tried to connect with second DP cable i have but it still doesn't work.

i also connected the computer to another monitor with DP cable and it works fine.

please advise.

thanks.