In Analytics the Returning visitor statistics are calculated by comparing the currently selected period with the previous period.
If you choose "today" as a time period then today's data is compared with yesterday’s data to calculate the number of Returning visitors, i.e. the number of visitors who visited the site today and who have also visited the site yesterday.
If you choose the complete time period for which visitor behavior has been tracked, then there is no data to compare with, so the number of Returning visitors presented is zero. Therefore, if you don’t see any Returning visitor statistics for your website, reduce the span of the selected time period to less than that of the complete tracking time period.