HELP me please! What's the cause..........?

There is possibility that someone else has hacked your account and changed account setting. What is your primary email address in that account you can write to customer service request them to send you another password to your primary email address. Spartan

You might have typed your password wrongly. Since you can't see it, you can't be 100% surely you have done it right. Type it out somewhere else to see whether you have locked the key for Capitals when your password are all small letters, or the other way around. If you are totally sure you have typed nothing wrong, the only way is to contact Yahoo Customer Service. Or, you can change your password.

Wait for yahoo to get over the glitch it is in.

Its possible someone hacked into it. I cant think of any other reason it may be doing that though. Maybe someone else knows more.

ring the service provider say BT they should be able to sort it out for you

possibly your keyboard was upside down and you ended up typing backwards?

possibly the caps lock were on.