P2715Q, no signal from DP or mDP?

hi

My P2715Q just arrived on 4th, the monitor works fine before yesterday.......

I turned off the monitor (but my computer will sleep after 30 mins)

Yesterday, when I turn on the monitor, its show:  Entering power save mode

I tried to restart the computer, but no help. I changed a miniDP to DP cable, no help.
I buy a new DP to DP cable today, but still no help
But nothing wrong with HDMI to HDMI cable!!!

I am sure my display card is not faulty, because I borrowed a display card from my friend and test, same issue.

I called my local Dell hotline, but today they are not working

What can I do now?

THANKS!