I would not waste a lot of time considering a one day experience on anything. There could be a technical issue or something else. Even a football game! It might be helpful to figure it out and it could be something important as a matter of fact (such as having a poor host who had your site down for hours), but if you spend a lot of time on something like that you can hurt yourself by what you do not get done. If it pops back the next day you are OK and may never know.
However, in my limited observations relative to swings like this I often see a fifty percent drop on occasion from high points, twenty-five to thirty percent from average, but almost never a total absence of traffic if it was of any size before. If a site is just starting and traffic is really low (15-20 a day) it would be totally possible to have a day with no visits.