I have signed out of yahoo, but it still says-you are already signed in. I have signed out of yahoo, yahoo mail, my yahoo and bt yahoo.

You're asking in the Yahoo mail category. Are you unable to access Yahoo mail or BT mail?
If it's BT mail, and you've switched from BT, you're likely out of luck as you would no longer have authorization to access their servers, as those would be restricted to BT servers. This is why independent email services (such as Hotmail, Gmail and Yahoo mail) are nice - they're not tied to any one internet company and can follow you no matter how many times you switch.

You can sign in to your BT Mail account either through BT Yahoo Mail or by using My BT. To know how to do this, you can refer to the instructions mentioned on the following link:
http://bt.custhelp.com/app/answers/detail/a_id/44907/~/bt-mail%3A-logging-in-to-my-webmail

GuruAid.com

Call 1-800-318-0612
It is a dedicated number for peple who cannot access their account

I think you have be a BT customer to get a BT email (@bt.com. @bt.co.uk
see https://uk.answers.yahoo.com/question/index;_ylt=A9mSs2LGwJJUOQUAvY5LBQx.;_ylu=X3oDMTE0bzYzNmF0BHNlYwNzcgRwb3MDOARjb2xvA2lyMgR2dGlkA1ZJUFVLMTdfMQ--?qid=20080923095717AADSN23
(could be outdated)

This from the bt website :

Log in to BT Yahoo Mail

Use BT Yahoo Mail? You'll be moving to the new BT Mail later this year.
If you receive a message asking you to change your password, it's important you do so.
changing their mail service, but I don't think this changes what I have said above.
18.December.2014
BT ar

It is doing that with everyone I'm afraid. BT has launched their new mail site and this will keep happening until they get it right. You can go to yahoo by clicking on the shortcut at the top of the BT page.

it shows invalid mail id or password

please keep on the option "Remember my ID & Password " or the option "Sign in Automatically"

you have to sign out so he can sign in.

In case your bt yahoo account is tide up to their corporate e-mail good chance that they might hold it. Your best option is to export all the mails that you need to keep and your address book.

Open a g-mail if you don't have one, they have a good size for FREE.

Once you sign up, forward all the mails and all your contacts to that new e-mail account.

Make sure you read their TOS (Terms of Service) to make sure that they can't hold anything against you.

Hope this helps,

J-inno

When you sign into Mail first, the new Yahoo mail automatically signs you into messenger. Mine did that all the time so now I sign into Yahoo Messenger first then when it tells me I have mail, I click on the envelope and it takes me to my mail account without signing me out of messenger. I hope that helps you.