Now that I tried to install a new router my computers are all screwy!

I'll admit it. Im not very good with computers...especially when it comes to networking. I don't know jack about it. But I tried installing a new router and to my delight, nothing worked. I tried rearranging all of the wires in various ways but it never works. So I went back with the old router hoping that I could atleast get a connection for a while(I'm doing this for a buisness so internet connections are neccesary. There is no IT guy either...we're a small buisness. So I have to figure this crap out or the rest of my company is gonna get flustered. Lucky me to be the guy who has to figure it out huh?). And guess what. Now only 3 of the 4 computers can get online. Unfortunately, the one that is off is the one that we need more pressingly to do UPS shipping.

Here's where I am confused though. All of the computers are hooked up the same. The modem goes to a router where all of the computers are then hooked up to(via the wall). So it makes no freakin sense for the one to not work. I've tried switching cables and I've tried switching which port in the router they are plugged into. For whatever reason the 1 computer will not connect. Then I go to that computer and check its network status and it says it is connected at 100mbps(or something like that). So what should I do?