You may find yourself logged out of Google Drive for Desktop for any number of reasons. If this does happen, logging back in is quite simple.
Click the start button in the lower left corner. Scroll down to Google Drive
If the sign in window does not load on the screen then click down in the lower right hand corner on the Google Drive icon. Choose Sign in.
You maybe asked for 2-step verification.
Please also input password when prompted.
Once you have completed logging in you can then access your files in the drive.