We observed that there are some Agents in a "suspended" state since 3rd of December.

We have both windows and ubutnu agents, but this has happened to only windows agents.

It shouws: 

Node is being deleted by Jenkins after idle timeout

 In the "Node Logs" it states: 

Agent successfully connected and online
Connection terminated

In the jenkins master "System logs" it displays:

AzureVMAgentCleanUpTask: cleanVMs: node wautosca337e62 blocked to cleanup

When I checked the portal, the VM is there alongwith NIC and Disk.

However VM Status is set to "Failed"

If states: 

Some details about the installed extensions are unavailable. This can occur when the virtual machine is stopped or the agent is unresponsive.
 
Additional error information is available for this virtual machine:
 
GENERAL
Provisioning state            Provisioning failed. Unknown network allocation error.. NetworkingInternalOperationError
0
Provisioning state error code        ProvisioningState/failed/NetworkingInternalOperationError
Guest agent            Not Ready. VM Agent is unresponsive.
 
DISKS
wautosca337e62_disk1_de2ae75276514eeb84aedadded60a91e 
Provisioning succeeded.
 
EXTENSIONS
customScriptwautosca337e62                        Unknown
 

Rant the following Powershell:

Get-AzureRmVM -ResourceGroupName $RGNAME -Name $VMNAME -Status

The output is attached: