Since your NetworkManager state is now DISCONNECTED, the only way to connect to you instance is interacting with the serial console
There is high chance that you did not assigned a local password for a user on a virtual machine instance in advance and stuck here but no worries! you can add follow this instruction to overcome this issue also:
Step one, create startup script in cloud shell using the command:
$ nano startup.sh and copy the following into it.
#! /bin/bash
echo "password" | passwd --stdin username
(where password is the desired password, must leave quotation marks. And username is the desired username)
Press ctrl+x and then y and then enter to save and exit
Step two, use the following command to insert startup script into your instance.
gcloud compute instances add-metadata example-instance
--metadata-from-file startup-script=path/to/file
(where example instance is your instance name and path to file is where the startup script file is located, in the case that you have not changed directories after creating your file the path would be ./startup.sh
Step three go back to vm instances in your console and restart your instance. Once it has spun up, click on the [Connect to serial console] button.
Once the startup process has finished, you can enter your credentials. You may need to push enter to get the login prompt.
Once logged in we can verify your hard drive space to confirm the log entries using the command:
df -h
Also you can use following command in your script to delete temporary file in order to cleanup your hard disk:
#! /bin/bash
rm -rf /tmp/*
rm -rf /var/tmp/*
rm -rf /usr/tmp/*
rm -rf /var/log/*