Spear & Jackson GCS400 N electric chain saw chain slips?

Hello, my Spear & Jackson GCS400 N electric chain saw was working pretty well for cutting up 3 branches of an oddly grown eucalyptus in my garden. However, I had to stop the work since for some reason it makes a kind of whining noise and the chain stops turning. The noise and what I had read about, makes me to think it's a slipping clutch but I would be grateful if someone could enlighten about the trouble and how to mend it. I would appreciate if the required spare/tools could be recommended, as well.

Thanks in advance

Alex