What that means, is that the BIOS didn't find any kind of local (in the computer) boot device, so it started looking for a network source to boot, and didn't find one of those either.
So, there's an excellent chance that somehow or another you didn't reconnect your HDD. There should be two cables coming from the back of the HDD. One power, one data. So if you only see one, that's a problem. If you do see two, then be sure to trace the smaller one (data) all the way to the motherboard and make sure it's firmly connected at both ends. Also couldn't hurt to give a small push to the power connector either.
<span id="INSERTION_MARKER">I asked around about PSU's and Video Cards, went and made the purchase, and decided that I'd never learn how to install them, if I didn't try... Well there we go... I removed my old PSU, that was easy... trying to pay attention to where all the cords went was the hard part. Installed the new PSU, tried to plug it in to all the same spots. Installed the new video card, easy enough. Now..... It won't boot up. Could this be as simple as not plugging something in with the correct power cord, or missing a plug-in altogether? I've tried to look at all the connections, just not exactly sure what I'm looking for. All those extra cords and slots all over the place is a bit overwhelming. I don't know that it matters, but the computer is a Gateway LX6810-01 PSU is a Corsair TX 650 Video Card is Gigabyte GTX 460 V2 I looked around at that problem, then PXE-53 No Boot Filename Recieved, and basically it seems as though it could be a ton of different things.... Just trying to get some other opinions before I take it to some computer guy and just pay to have it fixed.