Hello,
As @ Saikat mentioned, the robot is running as as service only for the enterprise version. If you close the robot tray, the service still runs.
If this service is stopped, you cannot run any flows.
If you have the community version, when you start the robot tray, a process called UiRobot or UiPath.Service.Host (for 18.2) appears under Processes tab from Windows Task Manger. In you close the robot tray, the process disappears, but if you trigger a job from Studio, it reappears.
@Th_Dam_TVan
Sometimes, because of windows restart or power on, the service can be slow to start. It will start eventually, as it is on a delayed start, but it may take a while.
However, this is rare from what I have seen.
P.S> I also like Misfits 
-Andrei