Post can’t be read after upgrade

Greetings. I just upgraded my blog to 2.6 and I did followed the instructions on wordpress but after successfully updating my blog, everything seems to be ok, all my latest post are still shown on the main page but when I clicked on each one to read them, an error message shows ” ERROR 404 – NOT FOUND ” – I checked on my dashboard and the posts are still there. I republished it and even changed the dates but the problem is still there. I made a backup of my database before upgrading.

Help anyone? Thank you