if (select count(*) from FROM [ReorderS]) > 0  

IF EXISTS(SELECT 1 FROM [ReorderS]) --what is your condition?
  BEGIN
DELETE FROM [ReorderS]

INSERT INTO [ReorderS]
SELECT ITEM,NULL FROM [INV_ITEM_MAS]

UPDATE [ReorderS] SET Quantity = [PO_Qty]
FROM [PO ITEM INTERFACE] INNER JOIN [INV_ITEM_MAS]
ON [PO ITEM INTERFACE].[PO_Item]= [ReorderS].ITEM
  END

declare @TheCount int
select @TheCount = count(*) FROM [ReorderS] where Item is not null
if @TheCount> 0 
  BEGIN
    --do stuff
  END
ELSE
  BEGIN
    --do other stuff
  END

if EXISTS(SELECT * FROM [ReorderS] where Item is not null)
  BEGIN
    --do stuff
  END
ELSE
  BEGIN
    --do other stuff
  END