Windows

Occasionally it helps to make sure the monitor is detected correctly. Try manually selecting the appropriate driver for your monitor.

I am sure there are other possibilities, I just can't think of any.

Under software control from your Desktop.

Currently you are only seeing the top right hand portion of your desktop, but surely there is enough of the Desktop to right-click on it?

Right-click on the Desktop and access your 'Properties' then click your 'Settings' TAB.

Now, if because you can only see the top right hand portion, you don't get to see the Properties dialogue box when it opens, then you'll have to be slightly more creative.

Can you fire up the 'puter in Safe Mode, which will boot you into standard VGA graphics mode, which should hopefully reduce the resolution and let you see more of the Desktop. Then, perhaps you can get access to the display settings to adjust for a better view.

By the way, what type of monitor is this external unit? If it's a CRT monitor there will be a multitude of controls on the lower front or under the front of the screen, that will allow you to squeeze the sides of the screen to make them visible again, or to swing the entire Desktop image from one side to the other - this might give you access to the Properties dialogue box also.