I have a mid-2011 Thunderbolt 21.5" iMac. A secondary monitor is connected to the iMac with a Mini DisplayPort-to-HDMI cable (a single cable, not an adaptor and cable).
When I was running Snow Leopard, the monitor worked just fine. After upgrading to Lion, the second monitor just has a black screen, and quickly goes into power save mode.
Has anyone seen this problem? Any other suggestions for solutions?