Snapchat feature availability can vary depending on your region and app experience. In some Snapchat experiences, the number of days in a Streak may not appear next to a friend even when you are still Snapping each other. If you continue exchanging daily Snaps, your Streak may still be active even when the day count and emoji are not displayed. If you and your friend missed the window to send each other a Snap, the Streak emoji may disappear and the Streak may expire.
You can read more about how Streaks work and when they expire here: How do Streaks work and when do they expire?