Oh no! In the future, please check what time zone your account is set to and make sure you reach your goal before the clock changes to another day. The "Daily Goal" graph on the right sidebar of your home page should show you how much time you have left for the day.
If you know you were in the correct time zone, had a streak freeze and lost your streak anyway, here is what probably happened:
We have looked in to many lost streak claims of people who said they know they missed a day, but who thought the streak freeze should have kept the streak going. Looking into this is not an easy task, and we quickly noticed that 99% of the time, people lost their streak freeze on another, earlier date, and then did not have a streak freeze protecting them. We cannot give the streak back, so to prevent this from happening, please check the lingot store occasionally to make sure your streak freeze is still equipped. You can quickly check by clicking on your lingot icon on the blue top bar of the Duolingo main page.