Is your G: drive missing?
The G: drive is added automatically by Google's File Stream tool. It will only show up if someone is logged in, though, and will similarly go away if logged out. So if your G: drive goes missing, you may need to log into File Stream again. The File Stream tool can be found by searching the start menu or via the icon in the task-bar, accessed form the lower-right-hand corner of the computer (see the picture).