I had this problem a while back, loads of emails stored, a small amount of free space. Logged back in, in the morning - 100% free space. Contacted yahoo (next to useless, but hey free) Didn't get any further after posting on the forum. Still don't know what happened, never did get to the bottom of it. Haven't had any problems since. If the emails are important to you either save them or open up another account and forward them to it.