Eugene's observations

Step one: power off your modem / router for 10 seconds then reconnect and wait two minutes before seeing if the problem persists. This reset is often all you need to get back in action.

If not, locate the master telecoms socket and connect your router to the test socket underneath the fascia. If the bottom half of the fascia does not slip down you have an old socket and will have to manually unplug every phone, fax, Sky, security alarm, ringer and other device from the line. Now change the microfilter for a different good quality one. If the problem has gone away, one of your appliances or the internal phone wiring is at fault.

Your modem or router will probably have lights on (if there are none check the power supply). The two key ones are those that indicate detection of a broadband signal - synchronization, and the one that indicates whether credentials have been accepted by the ISP - authentication. If either of these are out you need to change the router to confirm that it is not at fault. If that doesn't resolve the issue then report the problem to the ISP.

The next level of diagnostics involves changing the connect string in the modem / router. You would typically access this from a browser by typing the default gateway address (gleaned from an IPCONFIG command in the CMD window) into the address field. First write down the current username. If you change your username or connect string (NOT the password) to bt_test@startup_domain then restart the modem / router you will be cutting out the ISP's equipment, so if a connection to the BT test page is possible, any problem will be with your ISP's equipment.

If you connect successfully you can determine whether your computer software (e.g. browser settings, firewall & DNS) are configured correctly by going to http://www.bt.net/digitaldemo/ If you can view this website then your software and hardware is configured correctly.

To check whether an authentication problem is due to a bad username or password, you can change the connect string to a generic one for your ISP, e.g. bt_test_user@plusdsl.net If that connects but your own credentials fail, you need to ask the ISP for the correct details.

If you have a connection but the speed is slow, then websites such as http://www.mybroadbandspeed.co.uk/ will give you a rough idea of the speed, but a more accurate check on whether your service is below the wholesaler's threshold for accepting that a fault exists, cut out the ISP's equipment by changing the connect name (NOT the password) to speedtest@startup_domain once connected, visit http://speedtester.bt.com and run their test. Do this several times at different times of the day for a conclusive result.