This issue tracks back at least a couple of years now but I’ve just been asked about it again the other day. People report that on occasion Revit refuses to acknowledge when you click on either the Project Browser or Properties Palettes. For example this thread at Autodesk’s User Forum began in February 2014.
The suggested methods, in the thread, for fixing this issue include: Using Save As, Disabling Hardware Acceleration and clicking on the Help icon. One person posted that their screen went black first and then Revit crashed. That bit sounds like a graphics card/driver could be involved.
Those fixes resolved the situation but don’t tell us specifically why it happened in the first place. Since it has not happened to me personally I can’t say for sure why it happens either. I have heard that some errors generate a warning message that can get lost behind the Revit UI. Using ALT + Tab…