Yup I've had this happen a couple of times, specifically with @myyahoo.com emails... no idea what triggers it aside from perhaps just inactivity (but my normal @yahoo.com ones are fine) but I haven't been able to get around it either. :/ Very frustrating, I've heard Yahoo support requires you to pay as well for support services?? so personally I never looked into any official support recovery paths. But if it is an important enough email it could be worth pursuing!