Note: I can't take credit for these solutions, however a few of them have worked for me in the past.
Solution #1: Kill SystemUIServer
Launch ‘Activity Monitor’ (located in /Applications/Utilities/)
Locate the process ‘SystemUIServer’, highlight it, and click the red button “Quit Process”
Within a few seconds the menubar will rebuild itself and often Spotlight will magically work
Solution #2: Rebuild the Spotlight Index manually
Launch ‘Terminal’ (located in /Applications/Utilities/)
At the command prompt, type this exactly:
sudo mdutil -E /
You will be asked for your password, provide it, as this command requires administrator privelages to run
You will receive a confirmation message saying that index will be rebuilt
Wait until index is finished rebuilding, this can take a while depending on the size of your hard drive, amount of files, etc.
Note: You can also rebuild the Spotlight index with MainMenu, mentioned in Solution #4
Solution #3: Change Desktop Resolution
This is a strange fix but it works everytime for me when my Spotlight menu icon highlights but the search form doesn’t appear
Open “System Preferences” via the Apple Menu
Click ‘Displays’ and select a resolution smaller than what you are currently using, 640×480 seems to always work
Select your native resolution again, changing your screen resolution back to normal
The Spotlight search tray will magically become available again
Solution #4: Clear caches and preferences
Clear caches and preferences relating to Spotlight, this is best done through either the free tool OnyX, or the free tool MainMenu, I recommend MainMenu because it’s easier for novices
Note: Some users discover that just cleaning the Font caches is adequate to repair a screwy Spotlight, but it won’t hurt to clear them all
Solution #5: Reboot your Mac
This should almost be listed as #1 just because sometimes a simple reboot will fix Spotlight, but if I can avoid a reboot I will, and often, a reboot isn’t good enough
What do MDS and MDWorker have to do with Spotlight?
The MDS process and mdworker processes usually run concurrently on your Mac when Spotlight is indexing your Mac. Let the processes finish running to complete a relevant search index of your Mac filesystem.