1.Go to Following Navigation. PeopleTools>portal>General Settings
2.Check the DISPLAY SEARCH IN NAVIGATION BAR and DISPLAY SEARCH IN HEADER. And Save it.
3.Go to Following Navigation. PeopleTools>portal>Build Registry Search Index
4.Run the process by choosing any run control Id.
5.Go to process monitor.
6.Check if process completed successfully.
7.Restart the PIA
NOTE: It was tested on people tool version 8.49 and 8.50