Keyword search allows users to search for keywords across all primary entities. Keyword search box is available on Find screen of all primary entities.
The New Stuff: Customizing Outbound Call Lists
Have you ever seen Keyword search box is not visible on any Find screens?
Basically keyword search depends on the Sage CRM indexer service which we understood after referring Jeff’s blog. Here is the link.
https://community.sagecrm.com/partner_community/b/hints_tips_and_tricks/archive/2009/12/21/how-to-get-the-indexer-service-keyword-search-running-again.aspx
Here we observed that FullText.ix folder was missing from Sage CRM Library folder. Hence we followed 3^rd approach mentioned. Here we also observed after running the same, we encountered the below warning message on Find screen. However Keyword search box was enabled again.

Message indicates that re-indexing process is currently in progress. Keyword search won’t be functioning until message is displayed on the screen.
By following above steps one can easily enable keyword search box again on the Find screen.
Also Read:
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3) Control tab display using multiple SQL clauses
4) Flexible Grid Size in Sage CRM
5) SSA field behavior for custom entity in Sage CRM