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#11
The next time you get in a situation like that, hold the ctrl key down and roll the scroll wheel on your mouse down. Doing the same thing but scrolling up will Zoom. ctrl and the 0 key returns your screen to normal.
It sounds like you set your resolution to one that's not supported by your monitor. To fix it you need to go back to the recommended resolution of that monitor. One way out may be to try a different monitor, or even a TV if your using HDMI. In the old days you'd just boot up in safe mode standard VGA low res and reset it to the correct value. I think its a little trickier now. Have a look at this, Boot into Safe Mode on Windows 10