When the motorcycle is unused for a period of time (which can range from a few weeks, to a winter), moisture can form on the clutch plate and friction plate which can cause the clutch to stick. The symptoms are that the clutch lever on the handlebar, the clutch cable, and the clutch lever at the gearbox feel relatively normal, but the clutch itself does not disengage when you actuate the lever.
There are several techniques for freeing a stuck clutch.
1. Sometimes rocking the bike back and forth in gear (try several different gears) may free the clutch if it is not stuck too bad. If the bike has been sitting more than a couple of weeks, this may not work well and other techniques may be needed.
2. With the bike in neutral at idle, grab the brakes fully and drop it down into first while you hold the clutch lever in. The bike will jolt, and this may free the clutch. For safey reasons, be careful not to have the rpms much higher than idle.
3. With the bike on the center stand, jack the rear up so the tire is off the ground. Start the bike in gear, and with the rear wheel spinning run the revs up to 4,000 or 5,000, then pull in the clutch and step on the rear brake.
4. Ride the bike in 2nd gear, run up the revs per above and step on the brake per above.
5. With dry carbs (so it does not start), both brakes on fully, and the bike in gear, hit the start button. The movement of the drive train by the starter may break it free.
All of the techniques may take several tries, so don't give up on a particular technique if it does not work the first time.
When the bike is being stored for more than a few weeks, there are several things that can be done to reduce the likelihood of a stuck clutch, or reduce the severity of it. If you can periodically activate the clutch (especially when the bike if first put up), this can prevent the problem. You do not necessarily need to have the engine running; it is enough that the clutch is activated. Keeping the bike in neutral may reduce the likelihood of a stuck clutch, but this is not always going to work. However, if you store the bike in neutral, my personal experience indicates that it may not be as stuck as it otherwise would be.