Open two windows.
In the first window:
select password from dba_users where username='THEUSER'
alter user THEUSER identified by pass
In the second window:
sqlplus THEUSER/pass
In the first window:
alter user THEUSER identified by values 'password values from first query'
In the second window you will be logged on as the user. If the password can be set and reset quickly, this can be used for troubleshooting on a live production system.