You cant really get the SQL text that *caused* the block because, unlike the person that is *currently* blocked, there is no guarantee that the session that locked the rows is running anything.

They might have run 'update D set X=10 where X=2' two days ago, and have been sitting there idle ever since. Also, there is no "one" SQL that might have caused the problem. That session might have done:

update D set X=10 where X=2;

then

update D set X=15

then

update D set X=1
where not exists ( select null from emp where empno = d.x )

all of which will update/lock the row where x=2

v$session will tell you the blocking_session_id, which gives you information to either kill the session (or perhaps phone the person doing it!). Also, you can use resource manager to automatically kill a session that is idle but is blocking others.

https://docs.oracle.com/cd/E11882_01/server.112/e25494/dbrm.htm#ADMIN027