Understanding your local network and how to fix it could be quite important. Even with 24 hour support available on some ISP experience could be frustrating. Support can claim that everything is fine on their site and you will be stuck with your internal network problems. So being able to debug and fix certain network problems could be of a great help to you.
Network layout (topology)
First make sure that you understand your network layout. Most probably it will be one of the following:
a) Modem -> Router (Cable+Wi-Fi) -> PC
b) Modem (Cable+Wi-Fi) -> PC
In a second case Modem will work as a router and it is important to understand that in order to fix router related problems you will need to connect to modem administrative interface.
To get some details about your network type
ipconfig /all from cmd.exe launched under Administrative privileges. This command will show you lots of useful information about your network including address of the default gateway (for example we will use 192.168.1.1).
In case of layout a) this address will be address of your router. And in case of b) it will be an address of the modem.
Check connection to default gateway
Now type following command in cmd.exe launched under Administrative privileges.
Instead of 192.168.1.1 you should use default gateway address you got in previous step (ipconfig /all)
If you see replies then it means that your computer can see modem or router without any problems.
Also you can type http://192.168.1.1 in your browser and see if this will load router administrative interface. If yes then you definitely have good connection to your router and you don't need to look whether cable is bad or wi-fi is not working.
If ping command doesn't return replies then something is wrong between connection of that specific computer and router. If you have several computers check if all of them use the same default gateway IP address. If not - then something may be wrong with that PC configuration. Also in this case it make sense to check if you may have some problems in PC software of router hardware and consult support on that.