Warning "Index Server Down" was popped up when searching in Windchill PDMLink

The index search function is not available, an attribute search has been used. The search results may not contain all of the objects you expected. Please contact your Windchill system administrator.

ERROR [DataSource Task Executor-68] wt.indexsearch.AbstractIndexSearchDelegate doristsai - Failed to query Solr: 
java.lang.reflect.InvocationTargetException
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:606)
        at wt.index.SolrServiceLocator$PassThroughProxyHandler.invoke(SolrServiceLocator.java:137)
        at com.sun.proxy.$Proxy206.query(Unknown Source)
        at wt.index.SolrServiceLocator.query(SolrServiceLocator.java:189)
        at wt.indexsearch.SolrSearchDelegate.executeQuery(SolrSearchDelegate.java:254)
        at wt.indexsearch.SolrSearchDelegate.executeQueryList(SolrSearchDelegate.java:216)
        at wt.indexsearch.SolrSearchDelegate.performSearch(SolrSearchDelegate.java:737)
        at wt.indexsearch.AbstractIndexSearchDelegate.search(AbstractIndexSearchDelegate.java:261)
        at com.ptc.windchill.enterprise.search.mvc.service.IndexService.queryIndexServer(IndexService.java:216)
        at com.ptc.windchill.enterprise.search.mvc.service.IndexService.getPage(IndexService.java:142)
        at com.ptc.windchill.enterprise.search.mvc.service.IndexService.query(IndexService.java:126)
        at com.ptc.windchill.enterprise.search.mvc.service.AbstractQueryService.query(AbstractQueryService.java:70)
        at com.ptc.windchill.enterprise.search.mvc.service.WindchillSearchService.search(WindchillSearchService.java:116)
        at com.ptc.windchill.enterprise.search.mvc.builders.SearchDataBuilder.buildRawDataAsync(SearchDataBuilder.java:743)
        at com.ptc.mvc.components.ds.AbstractDataSourceComponentDataBuilder$ComponentDataSourceTask.run(AbstractDataSourceComponentDataBuilder.java:398)
        at com.ptc.mvc.ds.server.DefaultDataSourceManager$DataSourceTaskCallable.callInContext(DefaultDataSourceManager.java:226)
        at com.ptc.mvc.ds.server.DefaultDataSourceManager$DataSourceTaskCallable.callInContext(DefaultDataSourceManager.java:195)
        at com.ptc.mvc.ds.server.MethodContextCallable.call(MethodContextCallable.java:47)
        at java.util.concurrent.FutureTask.run(FutureTask.java:262)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        at java.lang.Thread.run(Thread.java:745)
Caused by: org.apache.solr.client.solrj.SolrServerException: java.net.ConnectException: Connection refused: connect
        at org.apache.solr.client.solrj.impl.CommonsHttpSolrServer.request(CommonsHttpSolrServer.java:475)
        at org.apache.solr.client.solrj.impl.CommonsHttpSolrServer.request(CommonsHttpSolrServer.java:249)
        at org.apache.solr.client.solrj.request.QueryRequest.process(QueryRequest.java:89)
        at org.apache.solr.client.solrj.SolrServer.query(SolrServer.java:321)
        at com.ptc.solr.service.SolrServiceImpl.query(SolrServiceImpl.java:283)
        ... 25 more
Caused by: java.net.ConnectException: Connection refused: connect
        at java.net.DualStackPlainSocketImpl.connect0(Native