It was "You have reached your question limit:Sorry, we are no longer accepting questions from this account. See the Help Center to learn more.".
I asked people to help me here:http://www.reddit.com/r/stackoverflow/comments/2ldq1y/i_cant_ask_questions_any_more_on_stackoverflow/ and on IRC . But doesn't help much. So I created a new ID and vote the old account's questions up. Now the problem is solved:
"Wait! Some of your past questions have not been well-received, and you're in danger of being blocked from asking any more.
For help formulating a clear, useful question, see: How do I ask a good question?
Also, edit your previous questions to improve formatting and clarity."
I believe the best way to solve a problem is help myself, not depending on others.