Try the following steps until the issue is resolved:
  1. Make sure that the letter has been created.
  2. Ensure that when saving the changes made to the merge document that you click "Save and Return". 
  3. Close all open Word documents and run the mail merge again.
  4. Make sure that the field you are basing the conditional merge upon is correct in step one of the Conditional Merge Wizard.
  5. Make sure that the records involved meet the conditions of one of your letters.
  6. The original mail template was a conditional mail merge and when following the steps of the Microsoft Word Mail Merge Wizard, simple mail merge was chosen. The letter created in as conditional mail merge does not transfer over to a simple mail merge even though the type of mail merge is chosen from within the same Mail template. So a letter created as a conditional mail merge would appear blank when merged as a simple mail merge. Try to merge the letter using one and then the other of conditional and simple merge forms.