In setting up OS X one of the convenient options is to add menu extras to the right-hand side of the menu bar, providing convenient access to system settings as well as functions provided by third-party utilities (virus scanners, activity monitors, etc). Sometimes, however, some or all of these menu extras may become nonfunctional and show the spinning color wheel when you mouse over them.
When the menu extras freeze, you may find that some (especially those provided by third-party developers) may still work. Additionally, some other built-in functions like Spotlight or the actual menus on the left-hand side of the menu bar will still function. You will also still be able to run applications, change system settings, and otherwise use your system without problems.
This problem happens because the SystemUIServer process, which is responsible for handling menu extras, has become unresponsive. Some menu extras (especially older ones) do not use the System UI Server and run independently, so if the server freezes they will still be functional. This is why some menu extras will freeze and others will not.
One thing to keep in mind is that the menu clock is run by the SystemUIServer, so you may be inconvenienced by it not being updated with the correct time and date. I know I regularly check the system clock, and at times when it has frozen I've found myself stuck at the same time until I notice something is wrong.
The way to fix this problem is to force-quit the SystemUIServer process, which can be done with Activity Monitor. Launch Activity Monitor and locate "SystemUIServer," which should be highlighted in red with "(Not Responding)" appended to its name. Select this process and click "Quit," and after confirming, the server should be launched again automatically by the system launcher. When this happens you should see the menu bar go blank and then be repopulated with the menu extras.
If the process will not quit, or if it does not restart and load the menu extras again, you will need to log out and log back in, or just restart the system to accomplish the same task.