First thing to do is to pull the dipstick and inspect the oil. Smell like gas? Is it contaminated with water? Is there any in the engine??? If it appears in good condition, does the kickstart lever rotate the motor? If so, then perform the 3k Service. If not, strap in for some work...
If it won’t rotate because the motor is locked up, pull the spark plugs. Get a quart of Automatic Transmission Fluid (ATF) and a bottle of Marvel Mystery Oil. Mix them 50/50, and drizzle it into the plug holes, directly into the cylinders.
Motors that have sat a long time can often become “seized” due to moisture rusting the piston rings to the cylinder walls. The mixture above, is great at eating the rust in a non-destructive way, and seeping past the rings to release the pistons. It will take some time, days in fact. Be patient. Keep refilling it. Don’t remove the motor from the frame as having it in there will let you use the transmission (put it in gear) and rock the bike back and forth to help free the motor.
All of this is only if the motor is seized.