Since the chain is on, you should rotate the engine by rotating the crankshaft, not the camshaft. But if it still won't rotate with no more than light-to-moderate turning force on the crankshaft, then your timing may be out. Is it in time now or not? (Of course you know not to force it to turn if you're getting solid "brick wall" resistance.)
If the engine is in time, then another possibility is that carbon deposits in the combustion chamber have swelled due to absorption of moisture while sitting this long. I had that happen on mine because when I replaced the valve stem seals, I used compressed air to hold the valves up while working on each cylinder and moisture in the compressed air caused that to happen (I didn't have moisture traps on my simple home compressor setup). I guess it's also possible for deposits to swell just from moisture in the air if it sits unused for a while in humid weather. If this is the case, I can talk you thru how to get the carbon deposits out without having to take the heads off.
Your timing being off is a more likely and simple explanation, so don't assume the problem is swelled up carbon deposits unless you are 100% certain the cams are in time.
So - the most important question is: Is your engine presently in time or not?
If it is out of time, you need to drop one or both cam sprockets to be able to re-time it. Depending on the positions of the cams relative to the crankshaft, you may have to piece-wise sequence the cams and crankshaft to get past piston-valve interference situations (trying to avoid your having to remove the camshaft bearing blocks to lift the camshafts out to re-time). It may help to know that if you can get the crankshaft close to any 60°-from-TDC position (the crankshaft sprocket mark pointing to the oil pump mark or any position 120° from that), you will then be free to rotate the camshafts (with the crankshaft staying in one of the 60° positions) to and thru any position without piston-valve interference.