So, 1 year and 340 days ago I bought a desktop computer.
Since then, it has served me well.
But lately, I wanted an upgrade, so I bought a new video card.
I documented myself about the compatibility, and it is okay.

So I opened the case, cleaned up that... dust elemental living inside of it.
Unscrewed the plastic thingie on the outside to unscrew the old video card.
Because of the stupid arrangement of the ports, I had to unscrew the motherboard to unplug it.
So I unscrewed it, removed the old card, put in the new one, moved the motherboard back, screwed it back in, screwed the video card on the holder... thingie, and screwed the plastic thingie back in.
Everything went smoothly, nothing had to be forced in/out.
I connected the external power supply, closed the computer case, put the tower back in it's place and all the cables back in.
When I pressed the power button, the LED turned... some color I can't distinguish. It stayed that way for a second, and then it went off.
I tried a bunch of things, including permuting the external power supply arrangement (1 connection, 2 connections and no connections), with no success.

And here are some of the specifications:
Motherboard manufacturer: Asrock
Processor: AMD Athlon II X2 3.0 GHz
RAM: 2 x 2GB (had only 1 initially, bought the second plate a bit later)
OLD video card: AMD Radeon HD 5450
NEW video card: Gigabyte nVidia GeForce GTX 650 GPU, 1GB GDDR5 128bit PCI-E, Dual-link DVI-Dx2 / HDMI / D-Sub
Power supply: 450W + all the requirements I managed to find on the internet are met (+12V 18A or something)
More specific information is stored... On that computer.

If required, I may open the case again and read the stickers to find more specific information.
I can also provide photos if necessary.

Any ideas? Suggestions? Something? :|