Just go to central administration of SharePoint.
Click ShareServices in central administration
click search  settings
click Content sources and crawl schedules
Click on New Content Source (If there is not exist people  search already)
 
Give its name [People Search]
in start address give
 
sps3://yourservername/
 
in crawl settings select (Crawl everything under the hostname for each
start address)
in crawl schedule set your crawling schedule
 
aslo check the (Start full crawl of this content source) option
 
and then press ok.
Now it will work.