In the Surveys app, view the survey you wish to pause.
You'll arrive on the "Overview" tab. At the bottom of this page is a "Suspend this Survey" button. Just click that and your survey will be put on hold.
To resume your survey simply click the button again.
Note: This doesn't extend or change the closing date, it just makes it unavailable to the public.
Anyone who tries to view your survey when it's paused will be shown the "Survey Offline" message that was configured when you set up your survey.