Thats alright! I don'th think that yahoo has a limit for messages (I've had that many messages for a few years) The mailbox was below 100% capacity. I know yahoo normally allow you to go slightly above this if a large mail comes in.
Its just infuriating to have a full inbox with emails that you want, then the next day nothing!
I've logged a call with yahoo. The mails only took up around 5mb of space. I've had loads of mails in the inbox for about 3 years now, so I can't see why it would suddenly become a problem. Why would it just clear the mail box?