Very likely your clutch is fine, just stuck as has been commented. Letting the bike get completely warmed up can help getting it free as has also been mentioned. The thing to understand is that this has nothing to do with any adjustment, but it is good to make sure that the pressure plate is allowing for as much free space within the clutch pack as possible. That means that you would like to push the pressure plate against its springs the greatest distance you can to allow for the most free space within the clutch, or in other words make the clutch pack as loose as possible so the plates can separate. So until the clutch is free, it can help to take all the slack out of the actuating mechanism, which is the throw-out mechanism and the cable and lever that operate it. Once the clutch is un-stuck, you can adjust the cable and throw out.
Aside from taking the clutch pack apart to separate the plates manually, all the methods to jar them free are pretty abusive to the drive line. That said, I have not experienced any actual damage, and I have seen this problem a LOT as it seems that this can happen to any multiplate wet clutch if it is allowed to set up for long enough. I have always had at least a few bikes that did not get used for too long and so have had plenty of stuck clutches. Most of the time they break free pretty easily - just putting the bike in gear while stopped. Or with the brake on hard if that does not work. I had a friend long ago that promoted putting the front wheel up to a wall putting it in gear. And of course center stand and rocking the wheel while the engine is off (clutch pulled in) or just repeated placing in gear while on the stand engine running which is no doubt the kindest to the drive-line but places the least impact on the clutch to knock the plates apart.
If it was me, I would do the following: I'd tighten up the throw out to make sure I had good movement of the pressure plate. That is the adjustment on the engine case, usually with a lock nut on an adjusting screw. Take the lock nut loose, then tighten the adjusting screw until all the slack is gone and then re-tighten the lock nut. You'll have to go back to adjust this correctly after the clutch is free. Then take all the slack out of the cable. You'll have to readjust this properly later. Now when you pull in the clutch lever, you will be sure to have the pressure plates clamping pressure sufficiently removed. It you still cant get it free, it is time to take the clutch side cover off so you can access the clutch pack. Look at the pressure plate while operating the lever. The pressure plate is the center outermost disk shaped casting on the top of the clutch assembly. You should see it move slightly - maybe 1/32" or so as you pull the lever. If you do, the plates are just stuck. If you don't, it is time to start taking things apart (throw out mechanism first) to find out why the pressure plate is not releasing. If it is moving, you can try to separate the plates manually by trying to pry them apart with a feeler gauge. Or you may be able to coax them apart just by pulling on the friction plate fingers toward the pressure plate one friction plate at a time. Obviously you have to have the lever pulled all the time while doing this. You should be able to get the top friction plate loose and move on to the next until all are loose. The friction plates are separated by steel drive plates, so if you get all the friction plates to move, each steel drive plate could still be stuck to the adjacent friction plate so only half of the assembly is necessarily free. Getting a feeler gauge between each friction plate and the drive plate on both sides of it (except for the first and last plate) will ensure that all the plates are not stuck. Alternatively, you can simply unbolt the pressure plate and pull out the friction and drive plates one at a time, inspect them, coat them with new oil and reassemble.
once you get the clutch sorted, you will have to adjust the mechanisms that operate it. Mainly that is the throw out mechanism. With the cable completely loose or unattached, loosen the locknut and turn the adjusting screw in and out until you can feel when it takes all the lash out of the mechanism. At this point, the clutch is completely disengaged and the engine and drive line are locked together. This would be perfect, except that as soon as a little more pressure is applied, the clutch will begin to engage and slip, which is fine if you are pulling the lever, but as the engine heats up/cools down and expands/contracts this could happen by itself so you need a little insurance. So back off the adjuster 1/4 turn to allow for a little space in the mechanism and then tighten the lock nut. Best to do this warm, and your manual may offer slightly different procedure.
Once done, adjust the cable. It can be any way you like, as long as there is at least some slack. If there is no slack, you risk pre-loading the throw out, which could remove the slack there which you created with the 1/4 turn. When that happens, you risk slightly engaging the clutch, allowing it to slip - maybe so slightly you don't notice, which ultimately will fry it.
