I have a ViewSonic VA912b 19" display I use as a secondary monitor. When I turn it on, the power light is green for a few seconds, and then switches to blinking green. The display stays black.
Windows thinks the monitor is on, as it shows up in the control panel as a second monitor.
If I unplug the DVI cable, it displays a "No Signal" message and the power light goes to amber, which means it went to sleep.
The solution was:
The monitor came back on at that point.