I've been struggling since ever with my DELL U3014 on the DisplayPort saying there's no signal after a power cycle of the monitor.
I just found a solution to bring the monitor back to life without rebooting the box; just ssh into and issue:
$ DISPLAY=:0 xset dpms force off
$ DISPLAY=:0 xset dpms force on
I'm running Gnome3 on Arch Linux, so YMMV.