Error with Activity in Projects

Hello everyone, 

I have met the error couple of days and don't know why it happened and how to solve this issue. 

In Projects, I didn't see the Activity above the tabs Project Plan. They are close or blocked. And when I click to edit the Green Icon / flag is gone but Activities are not displayed.

If anybody knows or has met, please share ....will be very grateful.

Best Regards, Kseniya