trouble sharing internet connection

Hi there.

I have two computers, one the host computer connecting directly to the internet through a D-Link DSL-200 adsl modem (via usb), and a laptop connecting through the other computer. The two are connected via a normal ethernet cable.

I have been able to share internet up until recently when I had to play around with some settings to be able to transfer files from one to the other. Now I suddenly can't anymore, even though I can transfer files from the host computer to the laptop. I noticed however that the laptop cannot ping the host computer, and the host computer cannot ping itself (i.e. localhost or 127.0.0.1).

Not installed or changed any firewall settings, so this should not be an issue. Thanks in advance