Press CTRL+ESC. If your Taskbar magically reappears, then it's probably simply been resized too small, repositioned somewhere you didn't expect it, or perhaps it's beneath another application that's running full-screen on your computer.
If CTRL+ESC didn't make it come back, then the program that displays the task bar most likely isn't running. That program is often called the "Windows Shell", but it's really "explorer.exe". It's the program that not only displays the Taskbar, but also responds to the CTRL+ESC sequence.
The workaround: press CTRL+ALT+DEL to fire up the Task Manager. Now on the File menu, select New Task (Run...). Type in "explorer" and press OK. That should restart explorer and re-display your Taskbar.