Ssh not working and server inaccessible (server down)

It looks like you've added AllowUsers in /etc/ssh/sshd_config configuration file.

To resolve this issue, you'll need to attach the boot disk of your VM instance to a healthy instance as the second disk. Mount it, edit the configuration file and fix the issue.

Here are the steps you can take to resolve the issue:

1. First of all, take a snapshot of your instance’s disk, in case if a loss or corruption happens you can recover your disk.

2. In the Developers Console, click on your instance. Uncheck Delete boot disk when instance is deleted and then delete the instance. The boot disk will remain under “Disks”, and now you can attach the disk to another instance. You can also do this step using gcloud command:

   $ gcloud compute instances delete NAME --keep-disks all
   

3. Now attach the disk to a healthy instance as an additional disk. You can do this through the Developers Console or using the gcloud command:

   $ gcloud compute instances attach-disk EXAMPLE-INSTANCE --disk DISK --zone ZONE
   

4. SSH into your healthy instance.

5. Determine where the secondary disk lives:

   $ ls -l /dev/disk/by-id/google-*
   

6. Mount the disk:

   $ sudo mkdir /mnt/tmp
   $ sudo mount /dev/disk/by-id/google-persistent-disk-1-part1 /mnt/tmp
   

Where google-persistent-disk-1 is the name of the disk

7. Edit sshd_config configuration file and remove AllowUsers line and save it.

   $ sudo nano /mnt/tmp/etc/ssh/sshd_config
   

8. Now unmout the disk:

   $ sudo umount /mnt/tmp
   

9. Detach it from the VM instance. This can be done through the Developers Console or using the command below:

   $ gcloud compute instances detach-disk EXAMPLE-INSTANCE --disk DISK
   

10. Now create a new instance using your fixed boot disk.

(I cannot comment to your question due to insufficient reputation.)

Is there some method to create a alternative login or some other options anyone can suggest?

I'm not sure whether that my understanding is correct or this answer will satisfy your question or not. I did a trick with ssh with pem file to GCE. The instance is required to have non-ephemeral IP address, let's say static IP address. As I can share this private key to my colleague (do it with your own risk of the leaking key).

Here's the steps to setup ssh with key file :

1. To create a new key in server ssh-keygen -t rsa -b 2048 -v

2. Follow the instruction including key location and key passphrase.

3. Append new key to authorized_keys cat /your/key/path/key.pub >> ~/.ssh/authorized_keys

4. Rename privatekey into .pem extension (optional)

5. Copy private key file to remote machine and set permission to 400 chmod 400 [KEY]
6. Test ssh connection with key file ssh -i [KEY(.pem)] [USER]@[HOST IP]

Edit: As @faizan indicated in comment if do not have serial console access to the VM, try adding the ssh keys to the ~/.ssh/authorized_keys using startup script.

That's it.

PS. Google compute engine create user per session if you not identified username. It depends on Google cloud SDK credential that you use. But log in with private key should work fine for ssh for any machine.