How to rebuild Magento Index while getting " Index process is working now. Please try run this process later."

You may get this error while rebuilding index on Magento.

Fatal error: Uncaught exception 'Mage_Core_Exception' with message 'Catalog URL Rewrites Index process is working now. Please try run this process later.'

If index request stop/timeouts/fails because of a PHP error, the indexer lock files lying around even though there's no indexing going on. These files are located in

/path/to/magento/var/locks/index_process*.lock

How to fix:

just delete the files and restart your indexing