Can't get 4k at 60hz (P2715Q)

Hi!

I have a DELL XPS 8700, a monitor P2715Q and a EVGA GTX 750ti card, which supports 3840x2160 @ 60hz at mDP 1.2.
It worked for some days when, suddenly, I turned on the computer and the screen was black. I've set the connection back to HDMI and it worked. But I need to use DP to get 60hz.
I spent a lot to buy all that stuff and finally get 4k 60hz. I'm really dissapointed.
Hope there's a solution.

Thanks!