|"...i whacked it in ..."|
Did you need to disconnect anything that was in the way?
If so, did you remember to plug it back in again?
You may have a loose connection somewhere inside the case.
Unplug the case/power supply.
Power off your monitor.
Check all the connections of the wiring to make sure they are all the way onto their pins and into their sockets, especially the main connector from the power supply. The wires close to the mboard going into the main power connector/socket should be more or less perpendicular to the mboard surface rather than at an angle. Unplug the main connector, plug it back in if in doubt. Most mboards require an additional connector from the PS be connected to a socket for power on the mboard, other than the main socket - if that isn't plugged in, your mboard probably won't boot. . Make sure all cards in slots are all the way down in their slots.
Some video cards require an additional power connector from the power supply be connected to a socket on the video card. If that is not plugged in, you'll probably get no video, and the mboard may not boot.
ATX power supplies are always powering ATX mboards in some places even when the computer is not running, including some contacts in the mboard's slots, as long as the PS is receiving live AC power, the switch on the PS is on if it has a switch, and the PS is connected to the mboard.
If you didn't remove the AC power source to the PS or switch it off, you can easily damage the video card and/or the slot circuits it's connected to while unplugging or plugging in the card.
Not removing the AC power source to the PS when you make any changes to connections or cards inside the case can also fry the power supply. See below.
Make sure the card isall theway down in it's slot and is fastened down with it's screw before you plug a monitor into it and boot the computer.
If in doubt, unplug the card, plug it in again.
If you didn't install the screw that holds it down before booting the computer, merely plugging in the monitor can easily cause the card to move upwards in it's slot.
Some PCI-E x16 and AGP slots have a lock at the end on the slot near the inner end of the contacts you can engage to lock down that end.
Try the previous card if the replacement card still won't work.
Not removing the AC power source to the PS when you make any changes to connections or cards inside the case can also fry the power supply.
Failing power supplies are common and can cause your symptoms.
Check your PS.
They often partially work, fans and hard drives may spin, leds may come on, yet you may get no video and the mboard will not boot all the way.
See response 4 in this:
If it is failing, you can usually replace it with any decent standard sized standard ATX PS with the same capacity or greater.
Standard (PS/2) power supply size - 86mm high, 150mm wide, 140mm deep, or 3 3/8" h x 5 7/8" w x 5 1/2" d , or very close to that, though the depth can be more or less for some PSs.
Don't buy an el-cheapo PS.
See response 3 in this:
Your power supply must have at least the minimum capacity required to support a system with the graphics card you are using installed, or the max graphics card you might install in the future.
(Onboard video - video built into the mboard - IS NOT A CARD!)
You can go to the video card maker's web site and look up the specs for the model - often under system requirements - the minimum PS wattage, and, more important, the minimum amperage the PS must supply at 12v is stated. If you don't find that, any card with the same video chipset including any letters after the model number has very similar minimum PS requirements.
Some power supplies have more than one +12v amperage rating - in that case you add the rated max amperages to determine the total +12v amperage rating.
It's extremely unlikely there's anything wrong with your mboard.