What happens when you try and input a number to store? Do you get a message displayed on the screen?
Have you tried the basics? Unplug the base station leave for a few minutes then plug back in, remove batteries from the handset, then place back in?
You could try a handset reset outlined on page 58 of the BT 4500 User Guide
or call the product helpline 0800 1456789
Thanks for getting back, I have tried unplugging base station for a few minutes, no joy. I have tried removing batteries and reinserting them, no joy. After re entering a number into the handset I just get a row of zeros. I have tried resetting, no joy. I am at a loss, I have rung the helpline number and was told to redial on a 0906 number, which will cost me 65 p a minute, no thanks they could have me hanging on costing me a small fortune. The phones were working normally before the power cut lasting no more than 3 hours.