If the other troubleshooting steps do not resolve your connection issue, you might be able to delete the existing instance and preserve your persistent disk.
Remount the disk to a new instance and make another troubleshooting attempt, or simply recover existing data from the disk.
gcloud compute instances delete [INSTANCE_NAME] --keep-disks=boot
gcloud compute instances create [NEW_INSTANCE_NAME] --disk name=[DISK_NAME],boot=yes,auto-delete=no