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It sounds like your external display is not recognized by your computer. I recommend this article to help troubleshoot the issue:
Apple computers: Troubleshooting issues with video on internal or external displays
No video or no signal, image distortion, "snow," or flickering
When using an external display be sure to check the following:
- If you're using an Apple notebook, confirm the AC power cable or adapter is securely connected to the computer and the cable providing power to the display is also secure. It is always good to have your notebook connected to AC power when an external display is in use.
- Confirm display adapters are fully seated in their respective connections and that they are supported models and for the computer and display. Refer to these articles to assist you with adapter compatibility and further configuration information:
- Remove all display cable extenders, KVM switches, or other like devices and retest to determine if the issue is resolved.
- Try unplugging the video adapter or cable and then plug it back in.
- If more than one video adapter is in use (or "daisy-chained"), troubleshoot by using only one adapter.
- Example: A mini DisplayPort to DVI adapter connected to a DVI to HDMI adapter is an unsupported configuration because there is a series of adapters in use.
- If available, try using a different display and or adapter (or use a different connector by using DVI instead of VGA, for instance).
Reset the system
You can reset the Mac's parameter RAM and SMC.Reset the resolutionStart by resetting the Mac's parameter RAM. If the display does not come up, was previously set to an unsupported resolution, and still results in no video:
- Start up in Safe Mode.
- From the Apple () menu, choose System Preferences.
- Choose Displays from the View menu to open the preferences pane.
- Select any resolution and refresh rate that your display supports.
- Restart your computer.
I'm experiencing the same issue with my MBP. It only happens when connecting to a projector, and I've tried both thunderbolt (using the DVI and VGA adapters) as well as the HDMI port, but the macbook pro does not detect the projector (I've tried 4 different projectors at work).
When I'm at my desk, I have both HDMI and one of the thunderbolt ports (using the DVI adapter) connected to monitors and have no problems connecting to extend my display.
What can I try?
2012 MBP running Yosemite