my yahoo emails have disappeared from my ipad and iphone6. My google email and my windows computers are not affected.

My Yahoo emails have disappeared from my ipad and iphone6. My gmail is not affected. My windows desktop is not affected. Only my Yahoo mail on my apple devices are affected. I tried deleting an email account and adding it back. When I tried to add it back, the message was that the account already existed. It does not show up on my list of email accounts. How do I fix this?