Why wont my computer send a video signal?

I turn off my computer, turn off the power supply and unplug it. Then I unplug my tv acting as a monitor into my friends computer, then plug the tv back into my computer, turn it on and it wont output video to any monitor I have outside or Safe and VGA mode. Ive tried changing the resolution but that does nothing. How do I fix this.Why wont my computer send a video signal?
reinstall your video driver.

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