Oh, well often this can occur when just viewing the post, but now I see you cannot even login…
I guess I misread and thought the post/page was not accessible…
typically a post itself should not cause you not to be able to login…
Are you by chance using a server caching plugin such as W3 Total Cache or WP Super Cache?
Since this is more likely related to plugins or your theme, please use an FTP Client such as FileZilla or your host provided File Manager to rename the /wp-content/plugins/ folder to old-plugins
Clear your local browser cache and try to login.
If still cannot login, make sure a default unedited theme is in place, and rename your in use theme folder to old-theme_folder_name (in /wp-content/themes/)
Clear your local browser cache and try to login.
If not that, do a Manual Update (just delete the two folders in root /wp-admin and /wp-includes) and NOT /wp-content (make sure not to delete or overwrite) replace from a fresh download of WP 3.5.1.
At this point you have fresh WP3.5.1 files, no plugins, default unedited theme…test…report back.
