Open up the Task Manager by pressing Ctrl+Shift+Esc or by right-clicking on the Start button and selecting "Task Manager." Click on the Processes tab. Locate and click on Windows Explorer in the list. Then, click on Restart in the lower-right corner.
You should see your Taskbar appear momentarily before disappearing again. Pressing the Windows key should bring everything back to normal. If that doesn't do it, restart your computer to see if that fixes the problem.