Hey, I found answer to my question!
When u reinstall the app u will nt get the streak the day u reinstalled but the next day onwards ur streak continues.
rupali hope u too got it back today.
BTW, thanks fr replying to solve my problem guys.
when its last day means when your 15th day of the streak , and when you complete the last lesson of your's strak's 15th day then in your notification bar 2 messages comes one of certificate and one of bts badge