The plates are just stuck together from sitting. This happens to many bikes of this era (and later!).
Run the engine to full operating temperature with the bike on the centerstand first. This will thin the oil and also let the engine attempt to relube the plates with some fresh, hot, thinner oil. This often helps to release the stuck plates. Then, dropping it into gear with the rear brake applied often breaks them loose.
BUT...if the oil in the engine was old oil and it sat for 5 years, chances are pretty good that the old oil had moisture (i.e. water vapor) in it, and that will have somewhat rusted the metal plates in the clutch. This gives them a real solid grip on the cork plates. Then you're going to be in for more of a fight, but it can still be broken free with some repetitive...'coaxing'...after about 100 miles that rust will be scraped off the metal plates pretty well.
Change the oil, too, and don't use oil with detergents in it. This will go a LONG way toward preventing it happening again. Detergents in the oil also have a nasty side habit of retaining moisture in the oil, in addition to making the clutch plates slip under power. The 750 suffers more from this than the 550 simply because the 750 has more torque with the same sized clutch plates. The 550 does not 'open' the plates very much with its lifter, so the oil will have a little harder time leaking in between the plates.
