A cloud-based task management app from Microsoft that helps users organize and manage daily tasks across devices.
I have the same problem, it seems to be a bug with multiple displays when I unplug a display and everything moves it resizes the side bar to 0 width I have on occasion been able to drag it back out but its very hard to find the exact position to do this,
I have found out how to fix this, maximise Microsoft to do then move the mouse off to the right until the mouse pointer changes then click and drag left, this then resizes the side bar if you do this when window is not maximised it just resizes the main window.