[support] Apache solr indexing issue

Dear All Drupal/Solr Experts I have a site with below details.

Drupal - 7.1.5 Apache solr 3

Users register their resumes and in cron task I used to index the resume. It was working good so far. Recently I observed that, documents are not getting indexed. I tried to index it fron config/apache-solr page. I am getting below error.

*An AJAX HTTP error occurred. HTTP Result Code: 500 Debugging information follows. Path: /batch?render=overlay&id=11968&op=do StatusText: Service unavailable (with message) ResponseText: * Can somebody kindly let me know what might be going wrong, why indexing is not happening and above error comes.

Thanks in advance. Kamal Net Cloud Systems Bangalore, India