Make Sure that your DSL Modem has the correct lights on.
Make sure that your connections are right.
Make sure the modem has power
The Ethernet cable must go from the Ethernet / LAN port on the back of your modem to your PC. If you have a USB Modem it should go from the USB port to your PC.
The phone line should go from your DSL jack to the Line / DSL port on the back of the modem
If the lights on the modem are NOT correct: Turn the modem off and unplug the power cord. Leave it off for 3 to 5 minutes and turn it back on. If the lights go back to normal try your Internet.
If the lights on the modem are correct: Turn the modem off and unplug the power cord. Leave it off for 3 to 5 minutes and turn it back on. When the lights go back to normal try your Internet. If that does not work try rebooting your PC and try again.
Trouble-shooting tips for dial-up
Disable Call Waiting while you're online.
Disable Call Minder while you're online.
Single phone lines
Make sure no other phones of fax machines are picked up during your online session.
Unplug any other devices from the phone line that your computer uses for the Internet.
Disconnect any other phones, faxes and answering machines from your Internet phone line. If you have a cordless phone, unplug it from the power point to see if that helps.
Unplug any other electrical devices close by (e.g. faxes, heaters or televisions).
Avoid running telephone wires close to speakers or other strong magnets.
Switch off transmitters like mobile phones, cordless phones to see if that helps.