I noticed that my 7 plus 10.1 stopped showing battery usage for last week (7 days). Before there were 2 options - 24 hours and 7 days. The second option just dissapeared out of blue... What could be the possible cause?
No- it disappeared because you just installed a new update; it resets the battery counter on these updates. You do not yet have 7 days worth of tracking under 10.1. 10.1 still offers this option, and it will appear once there are more days of usage to track.
Mine did this once too. Just wait a day or two and it should come back. Just another weird bug in iOS 10. Oh and it doesn't reset after an update. At least it shouldn't. Not the 7 days.