This happened about 3 years ago when I first started to attempt installing my own computer components. I bought a new R9 270X graphics card on Amazon and it took ages to come, so when it did finally arrive I just went straight to install it without really looking at how.
My existing graphics card was powered entirely by the motherboard, with no extra cables. I knew mine needed one so I picked out a power supply which was modular so it had an extra couple of PCI-E connectors. The new graphics card had two PCI-E connections, but I thought it only needed one. So that’s all I plugged in. Unlucky enough for me, it was the one that powered the fans and not the card itself. So when I booted the PC, the fans were spinning (which to me said it was getting power) but I was getting no signal on my monitor. I spent two days trying to figure out why it wasn’t working until I realised my stupid mistake, plugged in the other cable and sure enough it worked. Complete waste of two days I could have been playing games. Anyway, that it. 🙂