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I noticed that one of the fans in my computer was not spinning so I opened it up to have a look at it. I left it open so I could take the fan to a shop for a new one.

I started my computer, and it said "no input signal" on my monitor. I thought, "better put the fan back in", and I did so. However, even after I put it back in, the monitor still does not work, saying "no input signal".

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MY EYES!!! MY PRECIOUS EYES! – Hello71 Nov 7 '10 at 0:22
Which fan was it? processor fan? case fan? vid card fan? do you get any post beeps? any signs of power at all? Your system has stopped posting for a reason, but we need more information. Disconnect everything but the basics - power supply, processor + fan, vid card + fan, 1 stick of ram. Does it work then? try swapping the vide card or the ram, does it work now? Try the ram in different slots, etc. Test everything out. If you're not getting any power at all, change out your power supply. If nothing above works, swap out the main board. – MaQleod Nov 7 '10 at 0:35
If you're doing this with the cover off and your computer has an interlock switch (which is fairly unusual), that may explain it. If this is the case (no pun intended), putting the cover back on will solve the problem. – Dennis Williamson Nov 7 '10 at 0:48
@Dennis, good point, I didn't even think about that, I had a computer once with that annoyance. – MaQleod Nov 7 '10 at 0:53

The "No input signal" usually means that the monitor isn't physically attached to a graphics card. If you are using the "classic" connector (i.e. not DVI) I would check if the cable get loose.

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