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Is there anything else wrong ?
Did this happen after some event ?
- after you installed some software ?
- after you had some program remove malware from the computer ?
Where are you seeing this ?
On the main desktop screen, or in a Windows program ?
If it's on the main desktop screen, you may have a software problem that is preventing the main desktop screen from loading normally. Try loading Windows into Safe mode with networking mode.
Press F8 repeatedly while booting the computer, and when the Advanced.. menu appears on a black screen, choose Safe mode with networking.
If this happens in a program...
Windows has a full screen mode - you toggle it on / off by pressing the F11 key. You may have un-intentionally pressed F11
When that's toggled ON,
- In Internet Explorer, the bottom bar (taskbar and it's Start button) disappears at the bottom of the screen, and the bars at the top of the screen disappear but will re-appear if you move your mouse cursor to the top of the screen
- in My Computer, My Network Places, etc., the bottom bar disappears, and there is only one top bar.