The menu in Adobe Reader is very helpful and so it can be frustrating when you accidentally lose it due to a fat-fingering of keystrokes, etc.
Like most programs, hitting the F1 key will call up the Help Menu. From there, you'll have to dig around to discover that the way to get the menu back is to follow these steps: "F9/Shift+Command+M"
For most of us, this set of commands is confusing. I'll admit, it confused me too. The problem with the above command, is that it's really TWO separate commands - one for computers running Windows (F9) and one for MAC computers (Shift+Command+M). I suppose Adobe just assumed everybody would intuitively understand this.
Now you know. If you lose your menu in Adobe Reader, and are using a computer running Windows (WinXP, Vista, etc.), then hit the F9 key while the program is open. This should bring the menu back.
If you're using a MAC computer, you'll have to hit the three key-combination (Shift+Command+M) to bring back the Menu in Adobe Reader.