Mine is 'on' again as of 1015 this am.
During the meantime I tried everything.
I called a yahoo help telephone number and was directed by the person on the line to a site: logmein.com. He wanted me to put in a 6 number password which would allow him to get into my computer and check it out. I said "no". I think it was a scam. In the meantime I will start transitioning to gmail, unless yahoo can give me an explanation.