Okay, first: I know “disappeared” is not an adjective – but it conveyed my meaning, didn’t it? Next: I use the Spotlight menu more than the average bear. And, almost weekly it will up and disappear from my menu bar. I don’t know why it leaves, but here’s how to get it back:
1. In the Finder, select your main drive in a window’s sidebar, and follow the folder path: [your drive]/System/Library/CoreServices.
2. In the CoreServices folder, select the Spotlight icon and open it (Command-O).
That’s all. I haven’t found any benefit in restarting (nor does a simple restart seem to restore the Spotlight menu on its own); I am, however, impelled to advise you to restart anyway on general principles.