[dspace-install]/bin/dspace dsrun rdfizer --help
The online help should give you all necessary information. There are commands to delete one specific entity; to delete all information stored in the triple store; to convert one item, one collection or community (including all subcommunities, collections and items) or to convert the complete content of your repository. If you start using the Linked Open Data support on a repository that already contains content, you should run
rdfizer --convert-all once.
Every time content of DSpace is converted or Linked Data is requested, DSpace will try to connect to the triple store. So ensure that it is running (as you do with e.g. your sevlet container or relational database).