I have a Solr slave that is running in Tomcat. I added a core, so I changed solr.xml. To reload it I decided to simply restart Tomcat using the Windows Services managament conso
Clear the index directory and restart solr. It will work
Delete only write.log in data/index . Where is data directory is specified in conf/solrconfig.xml.
In case the other answers don't work, check your file permissions. In my case, I had to do a
chown -R tomcat6:tomcat6 /opt/solr/corename
Just deleted write.lock file under Solr > My Core > data > Index > write.lock file and restart solr server and it's worked.
Delete file write.log
in folder /data/index/
for your core then restart tomcat. It worked for me.