Let's start with the basics. Make/model of the router. Make/model of the broadband modem. Version/patch level of Windows in use. Has anything changed around the time things stopped working. Also, do this.
Turn everything off, modem, router, computer.
Turn on the modem, wait for a solid data/connect light.
Turn on the router, give it two minutes to get it's mind right.
Turn on the computer, see if you're connected.
If you're still not connected, do this:
For 98 or ME, Start, Run, COMMAND to open a DOS window:
--- or ---
For 2K or XP, Start, Run, CMD to open a DOS window:
Type the following command:
IPCONFIG /ALL
Right click in the command window and choose Select All, then hit Enter.
Paste the results in a message here.