Find field that was changed
SQL Server doesn't log the query text of an update. You can find the log records using 3rd party log reader tools (or fn_dblog), and you may be able to find out who did it (depends on authentication mode and how your users log in), and you can find the previous version of the data by restoring to a point in time before the update, but you're not going to be able to find the query after the fact. 

It may be time to start taking away rights, adding auditing (the full-blown auditing feature or your own triggers), or forcing data manipulation through stored procedures and rolling your own logging.