If you have the privileges, you can query the v_$session table for the SID, SERIAL#, osuser, username, status for all current sessions. Once you have identified the session that is causing the problem, if you have the privileges, you can submit the following command -
ALTER SYSTEM KILL SESSION 'SID,SERIAL#';
replacing SID and SERIAL# with the values from the query.
If you don't have the privileges, you will need to work with your DBA to do this.
There is another table that can show you what query the session is trying to execute, but I can't recall it at this time.