If you've used the F11 key to do a fullscreen (as versus the Maximize box in the upper-right window corner), there is no fix. This is a fact of life.
However, there are two other ways this could happen that ARE fixable:
1) The Taskbar may be hiding.
1. Look for a thin line on one edge of the screen.
2. Click on it. The Taskbar should open up.
3. Right click in the Taskbar and select Properties.
4. Uncheck the Auto-Hide the Taskbar checkbox.
5. Check the "Keep the taskbar on top of other windows" checkbox. (I'm betting on this one.)
6. Click OK.
2) The Taskbar may have shrunk or moved to another edge of the screen. To fix this:
1. Look for a thin line on one edge of the screen.
2. Right click in the thin line.
3. Drag it towards the center of the screen to make it big again.
4. If it's not on the bottom of the screen, drag and drop it back to the bottom of the screen.
5. Right click in the Taskbar, and select Properties.
6. Check the Lock The Taskbar checkbox.
7. Uncheck the Auto-Hide the Taskbar checkbox.
8. Click OK.
Hope that helps.