Is this a new battery? If not, start by replacing it.
If it is, pull the plugs, put the bike in fourth gear and see if you can spin the rear wheel by hand. Your answer will show itself quick.
If the pistons are stuck from sitting, with plugs out, squirt some marvel mystery oil into the cylinders and let it sit overnight.
Place a rag over the plug holes and see if you can rock or then spin the rear wheel while in fourth gear. If it breaks free, crank it with the button to spit out most of the marvel oil, then clean up the old plugs (if they don’t look wasted) and put them in. Change oil and filter, then see if it’ll start (or try to) using the button. Start there and let us know your progress.