Unhealthy robot would mean that the Robot Manager has received neither hear beat nor shutdown signal from the robot. Things to check would be
Is there is a connectivity issue from Robot to the PEGA platform?
Has the Robot runtime crashed (during an automation)?
For point 2, If the Robot runtime has crashed, you will not be able to re-launch the robot as PEGA recognizes that the robot has already checked-in and rejects the new login. I generally would find the corresponding operator record and delete it from PEGA [Not sure if there is a better way to reset the robot at this time]. This will allow the robot to launch again and PEGA will re-create the robot operator record upon Robot login.