Since this thread was never resolved & it was the first search result when I encountered this bug, I'm going to necro it with the solution. You can almost always fix this in vanilla KSP without going back to the space center.
First, open the debug menu with ( Alt+F12 ) or ( Ctrl+Alt+F12 ), then find the "Input Locks" tab and click the "Clear Input Locks" button.
This should instantly allow you to interact with the map menu again.
(Assuming the issue was caused by input locks and not one of the hundreds of other unpatched bugs in this game.)
