To answer that question you need a list of all the indexes that existed in the old situation and compare it with the current situation. You can get a list of all indexes with "sp_helpindex <tablename>".
But before digging into this you should run "update statistics" (or better, "update index statistics") on the table access by the stored procedure. Out-of-date statistics can potentially mess up query plans. Without up-to-date statistics, anything can happen so that needs to be ruled out.
If you don't have the "old" database around anymore then it may be difficult to compare the indexes that used to exist. But if you have a database dump of the old situation, you can access it quickly via the "archive database" feature and run "sp_helpindex".
HTH,
Rob V.