In general, there’s no way to find out a user’s password; all you can do is change it.
Any user with the ALTER USER system privilege can change another user’s password, like this:
ALTER USER system IDENTIFIED BY new_password;
To find out which users and roles have the ALTER USER privilege:
SELECT grantee FROM dba_sus_privs WHERE privilege = ‘ALTER USER’;
If that doesn’t solve the problem, you may be able to connect, without a password, as SYS (also called INTERNAL on some versions) directly on the database server. You may have to be logged in to the server as the administrator, or using the OS account that installed Oracle. If it asks for a password, just press <ENTER>. The details are version- and OS-dependent.
Oracle support can give you a much better answer.