Probably one of the big three calls I get from prospective clients. The first thing we try to determine is where the internet is failing. The first thing I do is look at the modem lights. The name of the lights can vary but they have similar function.
The first light that we look at is the DSL light. Sometimes it is labeled ADSL. This should be on and solid. When it is on this means that the modem has done all of the handshaking required to connect it to the local exchange. If it is off or flashing, then you have a problem between the modem and the exchange.
The first thing to do is to switch off the modem and leave it disconnected from the power for 5 minutes. Some times a power surge can temporarily scramble the modems brain. Reconnect the modem and see if the DSL light stays on.
If not then before we hassle the local phone provider we need to check the wiring in the house. Disconnect from the wall all phones, faxes and other devices that are connected via the phone. Also plug the modem straight into the wall bypassing the filter (also called a splitter). If the modem comes on and stays solid then there is a problem with one of the devices on the network. If it doesn't then borrow your neighbour's modem and plug it in. You don't need to load your settings as all we want to do is to see if it creates a solid connection to the phone exchange. If it does then replace the modem, otherwise call your ISP (Internet Service Provider) and get them to get somebody to check the lines.
On the other hand if your modem worked with all other devices disconnected then the problem is in your house. First plug the modem back into the filter (make sure that it is plugged into the ADSL side of the filter). If the modem was plugged straight into the wall don't worry as the phones need the filter not the modem.
If the DSL stays stable then plug in all of the other devices back into the wall one at a time. Ensuring that they are all connected to a filter. When you reach the one that causes the modem to drop out, the first thing to do is to check if it is the filter or the phone. If you don't have a spare (although after a few years most people accumulate a few), then try another one in the house. If it isn't the filter, unfortunately then the phone is for the bin.
Oh and by the way, don't be too surprised if you plug everything back in and it still works. A lot of times the problem is caused by a bad connection.