Thanks for your reply.
If you're accessing your email through your web browser your contacts list will be stored on Yahoo!'s server and not on your computer, so it looks like a problem at Yahoo!'s end. (I could never understand why BT teamed up with Yahoo!, since their email services have been plagued with problems for years).
This thread suggests that missing contacts can suddenly reappear again, so the problem
might simply go away in the near future:
http://forums.computeractive.co.uk/showthread.php?t=234652
(but see below about urgent action anyway!)
Since the problem isn't at your end, all you can do is to check that the missing contacts haven't somehow been added to the 'deleted contacts' list
http://help.yahoo.com/uk/bt/tutorials/cont/con_delcon2.html
and, if that doesn't work, contact BT AS SOON AS POSSIBLE. I've put that in capital because this page, about hacked accounts, suggests that there's only a very short period of time during which your contacts can be retrieved:
http://bt.custhelp.com/app/answers/detail/a_id/39411/
See 'Checking your email contacts haven't been changed or deleted' at the foot of the page).
Phone 0800 111 4567 (or possibly 0800 500 233, which I've seen suggested on another thread) STRAIGHT AWAY! (Lines are staffed 24 hours per day).