Not Using Index

We have a table in our database and an index defined properly on that table. SELECT queries on that table were using that index whenever executed. We did a REORG and after that SELECT queries on that table are not using that index at all. This caused a serious performance degradation on the database.

What could be the reason?

Did you change any parameters in init.ora?
Did you have table statistics in your data dictionary on the table?

Sergey.

is the index in UNUSABLE state?