Ahh.. Where to begin.

About 9 months ago, I put together a computer. About a week ago, I switched out the sound card to a new one, replaced my moniter with a projector, and cleaned out some dust. No big issue. Today, I noticed that one of my hard drives wasn't showing up in the bios. I thought this was a little odd, so I took the drive out, and placed it into a different computer, to see if it was a cable issue. The other computer recognized the drive alright, so I figured it might be some problem with the SATA cable. Using a new cable, I just placed the drive back into my computer; to my dismay, I now have both this drive (A 2TB samsung spinpoint "HD204UI") as well as a flash drive (A 128GB G.Skill SSD "FM25S2S") missing from my BIOS. This is very troubling to me, and I'm wondering if I might have a bad power supply or something. (Seasonic X750 Gold)

Does anyone have any idea as to what is going on?

If the hard drives spins into life then it probably won't be the PSU. Can you hear it/them spinning when you start the PC up? You might need to listen fairly closely.

This sounds like a motherboard (controller) failing. This would be a rare occurrence though, so here's what you can try first:

First make sure that both drives work fine in another computer. (Not just BIOS detection)
Then double check the connectors on the motherboard and the drives -- maybe they're just dusty/greasy -- and also try using different cables.
As a last resort unplug your computer from the power line and turn it on. Of course it won't turn on, but it will discharge the PSU and other capacitors. Then short the BIOS reset pins, wait a minute and put everything back together. Sometimes this can fix random errors.

Hehe... So I looked at it this morning, and realized that the SSD is because while installing the other hard drive, I accidentally unplugged it's SATA cable.

The Samsung drive seems like it was an issue with the cable as I am backing it up on another computer while I write this.