Check whether the network settings are correct in the PC which is directly connected to modem.
To enable internet sharing, the main PC (host) should be properly configured. Also, check the whether sharing is enabled in another PC (client).
Sometimes the problem can be just physical, hardware based - Check whether the cable connnecting two PCs is not loose.
If the problem is related ot OS and you do not know how to configure internet connection, you may also visit
Microsoft Help & Support page