Sounds like either your clutch is slipping or come lose or your cush drive nut is undone. I think you need to take the primary cover off for a look see first. Ron
Could be a number of things. What happens when you say slipping, if it feel like it's grinding gears (worn kick start quadrant and/or K/S ratchet). If it slips smoothly then look at cush drive nut on the crank shaft (fallen off or loose), broken primary chain or a clutch that isn't engaging.