Difference between days and streaks
I started a few months ago and got over 150 consecutive days. Suddenly "Coach enters" and I lose my consecutive days. Yes, I chose Coach, but I thought it was an additional encouragement. So now I have about 39 consecutive days. I weep excessively at the loss of 150 days, but I still love you guys at Duolingo. Would that one of the caring corporate owls would restore my joy that - alas - is floating out there somewhere.
Here is some information about how Streaks work so you'll have it for the future:
The app tracks individual language streak and the website tracks Overall streak. Streak freeze only works on overall streak. So, using it will maintain your website streak but not your app streak. The two will begin generating different streaks.
Additionally, you must be connected to the internet and have successfully gained 1 point before midnight. (Midnight is determined by the time zone you were in when you first started your account. Even if you move to a new location, at this time, your Duolingo Time Zone will not change to match your new one.) If you are working on a lesson, you must have completed the lesson before midnight for it to register. If you have completed a lesson offline, it will only count if you have since reconnected with the internet and had those points register with your online account.
What I don't know about, is why your website streak dropped to 39. If nothing I've described above rings true for you, my suggestion is to subscribe to the Troubleshooting forum, then edit this post and move it there by selecting "Troubleshooting" from the drop-down menu instead of "Duolingo".
I hope this helps! ^_^
I didn't think the coach streak affected the active days streak here... as long as you're doing Duo daily. (I've blown my Coach streaks without losing my daily activity streak, unless something's changed.)
Coach mode is a different type of encouragement. In coach mode if you practice a lot on Day 1, you can skip Day 2 and still be 'on track'. This may have been what happen to you, you didn't notice that you missed a day of practice because you were still 'on track', causing you to lose your streak.
Using Coach Mode will not affect your streak.