After updating my IPad, my iBooks download won't open. What can I do?

My son updated my iPad for me about a week ago. When I went to my iBooks, all my PDF and books had switch to have a colored cover with either a name printed on it or some jumbled word/number combo. When I click on them they just have the turning wheel going on and on, but I can't get to the actual information of the book or PDF. What should I do to get them in a readable format again?