innodb mysql无法更新，搜索，删除表中的某些项目

I am using PHP to update/insert/delete records in MySQL innodb. I have about 4 millions vcard data which are stored in the MySQL innodb. But there is a problem for only one recored.

For this special record, I can not update, select, delete that record by the primary key. The error return is:

ERROR 2013 (HY000): Lost connection to MySQL server during query

MySQL version: mysql版本:5.5.21-log

I do not even know what's the potential problem, any hint?

The table structure is:

CREATE TABLE vcard (
    username varchar(250) PRIMARY KEY,
    vcard mediumtext NOT NULL,
    created_at timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP
) ENGINE=InnoDB CHARACTER SET utf8;

The table is designed from here

The search sql is: select * from rosterusers where username = '3781353';

I can't comment because of my less reputation,so have to post in answer only.
This error appears when the query runs in an infinite loop. so please share your table structures and your query that you wrote.
Anyway error is probably in your where clause.
So,please share the information so that we can help you out with proper query.