There is a button to reset the document that says reset and reconnect. The button at the top says "Reset Entire Document". This will revert the entire document back to the way it was before. Please be advised if you have made changes to the other sections that you will lose those if you press this but you will get the introduction section back.
If you have made other changes that you want to keep, you can either copy them and put them back after you restore the document or you can not reset the document and just manually add the introduction section back by clicking Add a Stage and clicking the drop down building block button to fill with Introduction.