I have a Windows xp and i didnt use it for like 2 years so i thought i brush it off and repair it so i can play my old games again .. so i did just that and got a new video card and everything .. i plug it all in and IT WORKS (do a little dance) i played it off and on for a week and just a couple hours ago i turned my computer on and my screen was not coming on .. i thought it was a cable problem so i checked every cable and nothing was wrong so i unplugged it all and replugged it back in still not working but heres the interesting points i found .. once i unpluggd the cable going into my computer a little window came up and said "cable signal not found"
so i dunno what is wrong if anyone can help i will super thankful.

still no help ?

Remove the power cord
Hold the computer ON button in for 30 secs
Remove the addon Video card
Unplug the 20/24 Pin large power connector on the motherboard
Hold the computer ON button in for 30 secs again
Put the 20/24 Pin large power connector back in
Put the addon Video card back in
Plug in the power cord securely
Turn on

Wow ill try it right away thank you

Actually I'm about to sign off for the day
But, I'll wait ~ 10 mins I suppose

Edit

If it didn't fix the issue:

Could be bent or pushed in Monitor pin
Or faulty Monitor cable
Or faulty Monitor itself

Ideally try another Monitor (and also check the pins in the end of the data cable)

Let me know how it goes
Also I'm now assuming that the computer turns on? And even sounds as though its starting up?
If it doesn't, it may be your "new" video card died, or even you Power Supply (even if you get the power light on only)