I decided to plug in my external modem to see if that was okay, and it was. So now I have my laptop hooked up to my external modem and all seems to be okay again.
I have absolutely no idea how much it will cost to replace my laptop's modem, or even if it is possible, but I've decided to just forget about it and continue to use the external modem.
Now since there was nothing wrong with the wiring outside of my apartment, I'll get billed $60 for this "repair."