I am using Windows 7 64 bit running an Ivy Bridge with a HD7970 card form Sapphire.
I bought 2 U2414H monitors just a day ago and I set them up on dual monitor setup on display port. I did not attempt to daisy-chain them. They work fine right out of the box until today.
During start up BIOS information is displayed on both and once Windows login screen shows up one of the monitor goes blank. Windows and AMD Catalyst detects both monitor. But the blank monitor insist on going into sleep mode claiming there is no Display Port input.
I tried a variety of set up. Daisychaining and swapping cables, DP ports, right down to re-installing my graphics card physically and it's drivers too. Nothing worked short of reverting to HDMI in frustration.
What did the trick in the end was that I physically cut the power off from both monitor and rebooted my computer.
I suspect there is a bug in your power saving state relating to display port. Do take a look at it. If I manage to replicate the problem and solve it in the exact same manner I will post again in this thread. So this right now is only a hypothesis.