My modem wont let other ocmputers connect to the net whats going on?

ok so I got a new computer like 6-8 months ago and just got a switch the other day and tried to set it up today to let my computer and my old computer on the internet. it was not working and my firend tried to connect and that did not work so I thought it was the switch and I tried another one and that did not work so I went to the modem. It is a comcast modem ARRIS Model:TM502G P/N:TM02DH1G5 so I try pluging that into my old computer it does not work I try puting it in my friends laptop also does not work (it use to work on my old computer also it shows up as it being there but I can do repair or anything) and in my computer it works fine so what is the problem why does the modem only work on my computer?