Apache SOLR indexing stop working solr indexing has suddenly stop working.

Here is all the detail which i got from system admin

while running cron we are getting message during run cron.php through command

PHP Notice:  Undefined index:  REMOTE_ADDR in /var/www/XXXXXXX/includes/bootstrap.inc on line 1670
PHP Fatal error:  Exception thrown without a stack frame in Unknown on line 0

where should i look first and what are the possibility for apache solr indexing to stop suddenly.
The not to good solution was, deleted all the index of apache solr and tried with the fresh indexing and all of sudden all thing work well.