I'm running into a situation now and then when the Spotlight menu on OS X 10.9 can't be activated. When I click on its magnifying glass icon I receive only an error indication (beep tone) but get no chance to enter any search terms.
This can happen even if no programs have been launched and while the other menu items work as expected.
What causes this behavior and can it be overcome? (I have by now become reliant on Spotlight to open all kinds of documents, etc., so this seeming misbehavior is esp. unfortunate.)