// Get the report ID
    List <Report> reportList = [SELECT Id,DeveloperName FROM Report where DeveloperName = 'Count_Of_Cases_Related_To_Particular_Acc'];
    String reportId = (String)reportList.get(0).get('ID');
    
    // Run a report synchronously
    Reports.reportResults results = Reports.ReportManager.runReport('00O3D000000f6T3', true);
    
    // Get the first down-grouping in the report
    Reports.Dimension dim = results.getGroupingsDown();
    Reports.GroupingValue groupingVal = dim.getGroupings()[0];
    System.debug('Key: ' + groupingVal.getKey());
    System.debug('Label: ' + groupingVal.getLabel());
    System.debug('Value: ' + groupingVal.getValue());
    
    // Construct a fact map key, using the grouping key value
    String factMapKey = groupingVal.getKey() + '!T';
    
    // Get the fact map from the report results
    Reports.ReportFactWithDetails factDetails =
        (Reports.ReportFactWithDetails)results.getFactMap().get(factMapKey);
    
    // Get the first summary amount from the fact map
    Reports.SummaryValue sumVal = factDetails.getAggregates()[0];
    System.debug('Summary Value: ' + sumVal.getLabel());
    
    // Get the field value from the first data cell of the first row of the report
    list<Reports.ReportDetailRow> detailRow = factDetails.getRows();
    System.debug('########### detailRow'+detailRow);
    
The List(detailRow) is not fetching any data. Everything is working fine.