hmm,I didn't change anything about the database.
when I try to access bbs this morning (BJ timezone 5:30am), I found that there was a problem with database which make us not able to enter BBS. I tried to remove the link and title of the latest post, because I recognized that it was caused by a thread and I could still enter the admin page. however, it seemd that it didn't work. then I went to class at (BJ timezone) 6:00am, and was back till now.