My guess is that the index statistic became inaccurate. You may want to run UPDATE STATISTICS ... WITH FULLSCAN for that index periodically.

>I had an index like this on a table:
>
>

>   CREATE INDEX [IndexCombo1] ON [Reply] ([NoMember]) INCLUDE ([NoThread]) ON [Reply]
>

>
>For some kind of reason, this was not longer optimized. I had to delete the index and recreate it. What could have caused this situation?