Once the server is restarted, the indexer job starts re-indexing for all of the documents to migrate the index from Lucene 2.9.3 to Lucene 4.0 format.
Note: The re-indexing job takes 2 to 3 days to complete, depending on the number of files that need to be re-indexed. If you conduct a document search during the re-indexing job, you may receive an error message asking you to wait until the job is completed.