From the Page & Survey tab, scroll down to Project Questions and Dynamic Content & click Edit All. 

The questions in the example above say "Closed" in red in the bottom right corner, meaning no new responses can be submitted, only previous responses can be viewed.

Note: Re-opening all questions is most common, however, you can choose to Re-Open (an) individual question(s) if you do not want to re-open the entire question set.

The Edit All Questions in this set window will appear. If you would like to set a new close date (optional,) do that first. Select the green Re-open All button.

Once you exit this window, your survey questions will display the word "Open" in green, meaning they are open for particiapants to respond. 👍

Related articles:
How to re-open a closed question (when you don't want to re-open all questions)
How to un-archive a project
How to archive a question
How to hide a question
How to delete a question