I've not been able to post to my blog for several weeks, nor to comment (except anonymously) on any other blogs, and I've been too busy to discover why.
This morning I learned many people have had the same experience, suspected to be caused by bad cookies. I followed directions for deleting cookies and clearing the cache several times with no success. Then I did another thing, suggested by a top blogger help contributor: I unchecked the "keep me signed in" box before logging on. It worked!