It happens to me occasionally too.
Any problems I have had with BT Internet I call 0800 309080; the staff there are incredibly patient and extremely helpful.
A tip - when you are calling them to fix a problem use your mobile. It helps to be capable of connecting to the net whilst in contact with the man - you can then try the fixes to make sure they work. If you don't, and call back, you will probably get someone else and have to start all over again.