Sounds like the cam chain was not fully engaged with the teeth on the crank sprocket. Should be an easy fix. Remove the cam chain tensioner. Remove the cam sprocket cover. While maintaining upward tension on the cam chain so that it doesn't drop down into the crankcase, remove the cam sprocket. Engage the cam chain with the crank sprocket. While maintaining the engagement of the chain with the crank sprocket, engage the cam sprocket with the cam chain and remount the cam sprocket in the proper timing position. Replace cam sprocket cover, replace and adjust cam chain tensioner.

 

What Brian explains is easier done than it sounds, it is all about generating slack in the cam chain so you can engage the lower sprocket. Easier too if one first positions the TW vertical so that the cam chain will hang vertically. Vertical cahin then hangs in same plane as lower sprocket and thus readily re-engages sprocket teeth.

 

Don't forget to torque the cam sprocket bolt. More than one of us has been guilty of doing that. The little cylinder pin will shear and then you're screwed. Speaking of which, beware when removing the cam sprocket that you don't drop that dam pin down into the crank case. There's a hole down there that allows oil flow from the countershaft sprocket oil galley to return to the crank. Block it with a rag. When this happened to me, I got lucky with a strong magnet and from the bottom of the engine case (aluminum) was able to move it enough to grab with some mechanical fingers.

Also, it's easier to put the chain on the crank sprocket if the bike is standing vertically, not on the kickstand. Lean it to the right slightly and place it within close proximity of a wall. Rest the bar on the wall to keep the bike from tipping all the way over, then go for it. I use a pick tool to assist in getting that chain on the lower sprocket: