Chain came off, how do put it back?

Remove the cover. Loosen the two bolts that hold the chainsaw bar. Slide the bar back (it has a long slot for adjustment) until the blade can be lifted out of the track and sprocket. To replace the blade slip it onto the sprocket and the bar. You must hold the bar level while replacing the blade. Be sure the blade fits into the track around the bar. Once in place double check the placement on the sprocket and pull the bar outward until all the slack is removed from the chain blade. Holding firmly, (it would help to have someone assisting) keep the pressure on the blade and bar while replacing the bolt nuts. Then replace the hood cover. Smile.