Did you use the right type of ethernet cable? Depending upon the router & CM used you'll either need to use straight through (normal ethernet) or crossed (x-over ethernet cable).
The following page explains...
http://www.homenethelp.com/web/expla...-crossover.asp
You can buy both types of cable from any computer shop for a few pounds.
If it was working then stopped a common reason for this is accidently clicking the uplink button in/out on the router/switch, effectively changing the type of cable it needs to work. Check to see if you have an uplink switch and if so make sure it is set correctly...
http://www.duxcw.com/faq/network/uplink.htm
Good luck.