Can't connect Macbook Pro Retina to Dell P2715q 4k Monitor

I received a new Dell 4k P2715q monitor on Friday. It came with a mini DP to DP cable which I immediately hooked into the Thunderbolt port on my 15-inch, Mid 2012 Macbook Pro Retina. It worked beautifully on Friday all through the day. Was very happy with my purchase.

Took the laptop home over the weekend and brought it back today.... plugged into my thunderbolt port again and nothing.... Go HDMI to the monitor's HDMI and it works but only up to 1080p in 30hz which is no good. So the monitor still seems to be working but the Macbook won't recognize the monitor through the Thunderbolt (mini DP) port.

Any thoughts? Talked to Dell support which was worthless except to verify that the monitor didn't get fried over the weekend and also talked to Apple who suspected that it was maybe the mini DP cord that Dell sent with the monitor. I can't believe that a cable went faulty over a weekend when no one else was in my office.....

Thoughts? Thanks ahead of time!