What can cause a computer not to recognize a monitor connected to it

[KLIFF73]

what can cause a computer not to recognize a monitor and a keyboard connected to it? My OS is Vista Home BASIC

 

[Twoboxer]

Need to know your PC's configuration, or make/model.

Also, is this a new PC/build that never worked, or did it suddenly stop working?

In the interim, you could try disconnecting ALL USB devices/cables from your PC, including mouse/keyboard if they are USB. Then power up and let it go as far as it will, then power down via windows if you can, or by holding the power button in for more than 5 seconds if no mouse/kb.

Then plug in only mouse/keyboard and reboot. let us know what happened.