If the icons and taskbar are both missing, press ctrl+alt+delete-->start the task manager-->file-->new task-->type explorer.exe and press enter.
If just the taskbar is missing, move your mouse to the very bottom of the screen (assuming that's where it was), wait for the cursor to change, left click, and drag it up.
As cassandra mentioned, it it pops up, just right click on it, and unhide it.