Not yet faced the same but ...
There is a CLI script in moodlecode/search/cli/ called indexer.php
It has options/switches to command.
Options:
-h, --help Print out this help
-r, --reindex Reindex data
-f, --force Allow indexer to run, even if global search is disabled.
Takes web server out of the loop and will show errors that one might see cause 500's usually don't tell ya much.
Plus, depending upon how solr installed, one might find logs in
/var/solr/logs
that might be of help.
'spirit of sharing', Ken