If you have a "G:" drive on your Windows PC, then you most likely are running an application called "Google Drive" that creates the "G:" drive on your PC. When Google updates this application, it often closes and does not automatically restart (even after reboot)
The solution is to simply press your Windows Start button in the lower left corner of your screen that looks like this:
Then start typing the words Google Drive. You should see the following in the menu:Â
Run the Google Drive program and within a few seconds, your "G:" drive should return.