os x lion ethernet connection not working, help!!

Yesterday I lost my ethernet connection on my 2011 imac running lion 10.7.4. I had to switch to wifi, and now my ethernet connection displays as "not connected", as if the cable is unplugged. I have tried rebooting the router and powering the mac down & restarting and this doesn't help either. I don't know if this has any bearing, but this happened at approximately the same time my son had 2 friends over, and they were both using their laptops to connect to our wifi. Any help would be appreciated, thanks