If you have experienced a Solr crash during a commit (which, of course, is the worst possible time, Murphy`s law etc), you might have received a LockOptianFailedException when trying to start Solr. This makes it impossible to start Solr at all.
You can solve this by configuring Solr to delete any lock files when starting up,
you can also configure Solr to use a normal lock file instead, which you can manually delete if you prefer that.
Simply correct your solrconfig.xml to the following values, they are per default
commented out in solrconfig.xml.