You will have to take one side case off, and the cam/top cover off the head.
You need to figure how it came off though. If the chain is really worn they can come off, but they do need to be really worn, or, the chain slippers wear out badly, or, the most common cause(IMO), the ratchet on the chain tensioner slipped.
Top off and side case off, clutch off and some gears off, one chain slipper will pull out, the other one needs the head taking off to get out. Once the top is off you can look down and inspect the slipper that stays in. The tensioner is on the back of the cylinder. Take the chain out and check it for wear as per the book. Inspect the slippers for wear(grooves) or bits fallen off. Check the tensioner really carefully. Because the ratchet mechanism in the tensioner might just have a worn bit right where it had been sitting, but be good enough everywhere else, I'd replace it if in doubt..