Computer doesnt start after upgrading video card. ish?

so i installed a new fan and an xfx 8600GT graphics card. i pushed the power button, and the green LEDs lit up for a fraction of a second, then turned off. i pushed the power button again, and it didnt work. i came back about 3 minutes later, and it started just fine. can someone tell me why this happend, and how to avoid it in the future|||Justin your 8600gt graphics card may be to powerful for your mother board or your mother board may have played its last deck of cards depending on how old it is. i hope it didn't for your sake but i would first see if your video card is too strong for your motherboard first.|||the problem with the deivce not proeprly inserted or it was not recognised by the BIOS thats y u got the problem ther will be no issue once the connectivity good.