When you create a Sheet View Revit adds a titleblock to the view. What if you delete it and don't realize it till later. How do you put it back? How can you just put a Titleblock on an existing Sheet View? There is no command...Symbol? No... Component? No... Tag? No... Guess you can't?
Here's two options, best one first:
Use Edit menu > Copy to Clipboard to copy a titleblock from another Sheet view
Use Edit > Paste Aligned > Current View to paste the titleblock into the Sheet View that is missing the Titleblock. (Also can use Edit > Paste Aligned > Select Views by Name if you managed to delete more than one)
You can drag a Titleblock from the Project Browser (found under Annotation Symbols category) onto the Sheet View. Take care that you won't be able to position this titleblock in the exact location the previous one was. That's why the first option is better.