While subsequent Windows releases have changed slightly, all versions of Windows from XP forward have offered you the ability to customize your taskbar and "Start" menu. When customizing the tools, sometimes you might change things by accident. If you share a workspace with someone else, it's also possible that he's decided to play a practical joke on you and changed your taskbar and "Start" menu settings.
Your taskbar, and hence your "Start" menu, may be set to auto-hide whenever you're not using them. To see if this is the case, move your mouse cursor down to hover at the bottom of your screen. Wait a few seconds to see if your taskbar pops up. If it doesn't, repeat this process at each side and the top of your screen; it's possible to dock your taskbar in these places and auto-hide it as well.
Hiding Behind Windows
Unless you have your taskbar set to always be on top of other windows, sometimes your taskbar may hide behind other windows that you have open. Think of it like looking at a group of people waiting in line. From the side, you can see that there are quite a few people, but if you're looking back from the front, it's difficult to see them all, even though they're still there. Minimize all your open windows, or hold "Alt" and press "Tab" until you get to your desktop. You should then be able to see your taskbar.
You can shrink your taskbar or make it thicker, depending on personal preference. If you shrink it to a very thin strip, it's possible that you may not even realize that it's there when you look for it. If you minimize your open windows and hover your mouse over the top, bottom and sides of your monitor but don't see the taskbar, look for a very thin strip that stands out from your desktop. Once you see it, hover your mouse over it, hold down the left mouse button, and drag your mouse toward the center of your monitor. You'll drag the taskbar out of its shrunken hiding place and can adjust it to whatever size you want.
Change Taskbar Properties
Right clicking your taskbar pulls up a short menu that can allow you to adjust some taskbar settings, such whether or not it's locked in place. While locking it won't completely prevent someone with access to your computer from readjusting your settings as a joke, it can act as a deterrent, as any mischief-makers will have to unlock the taskbar before moving it. You can access more options for taskbar and "Start" menu display by going to the Control Panel in your "Start" menu. Here, you can choose whether or not to auto-hide the taskbar, attach tool bars for other applications and alter the display of the "Start" menu, to name a few available options.