My Wi-Fi internet connection on the desktop suddenly dropped?

Long story here:

I had internet on my desktop via Wi-Fi.
The Wi-Fi was from a router nearby.
The router nearby had a blue ethernet cord that ran approximately 70 to 100 feet across a ceiling to a modem on the other side of the building.

SUDDENLY, two days ago, nothing at all changed.
Except that I don't have internet anymore.

The error is that I do not have a cable plugged into a router (or else the cable is otherwise screwed up)

Now, this office/breakroom is pretty bad-a. It has an xbox with an expired gold membership, so it's kinda worthless, but it's there.

So, I reset the routers and the modem and did the turn off/wait 30 secs thing a bunch.
Didn't fix the internet.
I jiggled with the routers and clicked on a bunch of network connection windows and diagnostics a whole lot.
No dice.
Then I took the ethernet cord and plugged it into the desktop, and that didn't turn on the net.
So I thought maybe the culprit was the cord, but I recalled having trouble with direct connect to my laptop earlier, so Iran another test.
Since I suspected that my computer didn't like direct connections, I plugged the cord into the Xbox, and the Xbox live updated.
Since the Xbox Live updated, I figured that the cord was working.
So I figured maybe my router had kicked the bucket.
I bought a new router, hooked it up, and still the desktop won't get online.

Remember, NOTHING changed in my desktop of my connections between the time that I had internet and lost internet. So it's not like some unfixable setting took control of my computer and is rejecting the Wi-Fi.
But when I though about that, I pulled out my iphone, and I realized that even though it recognized the router signal, it was not connected to the internet.
I got my friend's ipad and had the same problem.
That means that the router works, both the new one and the old one, but there's no signal being put out.
Across the building, the modem, which is hooked up to a router for the main office, is all working perfectly. There's no problem at all with that equipment.
So it's gotta be the cable, right?

I have run every diagnostic. I have typed in every password I can think of.
The cable that will update an Xbox is plugged into a brand new router, yet my desktop will not pick up the Wi-Fi that it picked up without a problem a couple days ago.

Why?

My best guesses:

If the Xbox test an ineffectual method to determine the cable is working (which could mean that the cable is pinched or broken or dead for some reason)?

Is my desktop just a piece of junk, and something internal has to be fixed because it might spontaneously reject Wi-Fi, especially if it already doesn't seem to connect to a direct connection? (of course, the failure of my iphone and ipad to pick up the Wi-Fi signal disproves this theory).

Does anybody have any clues or suggestions or ideas?

Writing all this out makes me think it's the cable, but it';s a long, expensive cable that will be very difficult to replace. I don't want it to be the freaking cable.
'It's possible that, for whatever reason, the modem downstairs simply stopped communicating with the router upstairs. I don';t know why, and I don't know how to fix it if that's the problem.

I would suggest connecting directly to the modem and to the router and looking how they are configured. To do this goto start -> type cmd -> open it up -> type 'ipconfig' into the prompt -> look for where it says Default Gateway then look at what the ip is, usually something like 192.168.0.1, then type that address into your web browser's url bar (while connected to the router/modem) and you should see the configuration page. Now it will prompt you for a username and password. All routers have a different page at this point so you're going to have to just look around and try to figure things out. One possibility is that the Wi-Fi frequency isn't compatible with your desktop, maybe someone changed some settings remotely without you knowing.