Sure, you may be able to read a paragraph or two, or maybe even a page, but you don't retain anything of what you just read or have the attention span and/or will to go on. This is common for those who have ADD, are in possession of garbage literature, or are just so exhausted from having to read so many books during school/college that reading anything else, even for pleasure, has become impossible. To those who love to read, this is worse than heart disease and cancer combined.
The cure for reader's block? Just read blogs, and you won't notice.