Computer help please

[its hilly]

Tried to fire up my tower the other day and all sounded well, the screen lit up,
but then went dark, and the yellow standby light was on. I dug out my other tower I used to have at work and connected it instead and all works fine (fingers toes etc crossed) including the screen. Using all same cables, simply shoved them in the back of this instead.
I'm guessing that my other tower is not communicating with the screen. Does that mean my graphics card is dead? Could it possibly have anything to do with the fact that two or three days previous I had added a BT home hub to my system, and while this tower connects to it wirelessly, that one was connected by an ethernet cable?
I have checked for loose leads or connections without success.
Any clues anyone?

[Chef_uk]

Graphics card may just need resitting as they can come lose. Can't see how adding a router would cause the problem though unless you physically knocked the monitor cable. I would start with taking the graphics card out and resitting it. See how that goes

[Aberdeenplotter]

Graphics card may just need resitting as they can come lose. Can't see how adding a router would cause the problem though unless you physically knocked the monitor cable. I would start with taking the graphics card out and resitting it. See how that goes

If that doesn't work, try swapping over the graphics card from your old machine.

[bishop]

yer it sounds like video driver or the card itself, just wondering have you changed any of power settings on your comp , the monitor could be going into sleep mode, also does it do the same if you start in safe mode.

[oldie]

Are you getting any beeps when you boot up?