Q: My Macbook Pro after upgrading to Yosemite does not recognise external display.

Ever since I upgraded to Yosemite, I am unable to connect to external projector via thunderbolt / VGA adaptor. In the preferences, I have opened display, pressed option key and pressed Detect Display. Most of the time it hangs and nothing happens. As a result I cannot give power point presentation to my customers. Please help.