The likely cause of this is because the execution plan has changed.
Generate a current explain plan of the offending query and compare it to
a previous one that was taken when the query was performing well.
Usually the previous plan is not available.
Some factors that can cause a plan to change are:
What do you think the plan should be? Run the query with hints to see if this produces the required performance.
It can also happen because of a very high high water mark. Typically when
a table was big, but now only contains a couple of records. Oracle
still needs to scan through all the blocks to see if they contain data.
For detail, please contact firstname.lastname@example.org