After upgrade from Drupal 6 to Drupal 7 search does not work
Reindex the search data in config->search->setting and run cron again and again upto the index percentage reaches 100%.

This will solve your issue.

You can also do it via drush to get around limit on the number of items that get indexed per cron run.

$ drush help search-index


Index the remaining search items without wiping the index.
and/or

$ drush help search-reindex


Force the search index to be rebuilt.

Options:
 --immediate Rebuild the index immediately, instead of waiting for cron.