All of sudden it failed to start new workflows. Then I tried to start a WF manually.
Still It failed.
Then I have opened the "Workflow Manager Powershell" and tried to get the current status of the WF farm. (get-WFfarmStatus)
Surprisingly, the WorkflowServiceFrontEnd service was stopped.
With commands we cant start the WorkflowServiceFrontEnd.
First we have to STOP Backend service using,
net stop WorkflowServiceBackend
Then We have to START the Backend Service using,
net start WorkflowServiceBackend
Now we can check the STATUS again,
Both Services are RUNNING smoothly.
Now we can start Workflows.