Well you can still access BT Yahoo! Mail via either an email client or through your web browser, so it's not entirely clear how you're doing it.
I'll guess that you're using a web browser (such as Internet Explorer, Chrome or Firefox). If so, it's likely that the problem lies with that browser, rather than with your email account. So your first step should be to check that your browser is fully up to date. (With most browsers, you can do that from Help > About...).
Then try another browser. If you're using Internet Explorer, my question has to be WHY???? (It's dire!). Try downloading & installing Firefox:
http://www.mozilla.org/en-US/firefox/all/