First, open Device Manager and expand Sensors, from there right click HID sensor collection and Disable it. Then right click again to re-enable it. Once done, test to see if the issue is resolved if not continue reading. There should be a screen rotation lock, press it while the Yoga 2 is in the Tablet Mode and then test to see if it moves or not.
If you have attached any covers or external displays, remove them. Sometimes, these covers or externals displays prevent the screen from rotating automatically.
