wireless problems with the link between cable modem and wireless router

i have recently changed my internet service provider and as a result of this i have changed from adsl to cable. I am currently linking my cable modem to my wireless router. i have a pc wired directly to my wireless router and this works fine and a pc connected through wireless, the computer upstairs is connected to the router and says that i should be able to connect to the internet but it is not………if anyone could help me with this matter i would be much appreciatd.