My draw string or the crankshaft is stuck/jammed, What is causing this?

I have a six year old walking lawn mower, 6.0 HP Murray Select 22". Briggs/Stratton, model # 22315X8C, mfg. 01044, serial # 7428104413G519. Cranks by holding the handle bar then pulling the draw string. The problem I'm having is when I hold the draw string it will not bulge. Appears stuck. Get a feeling the crank shaft isn't turning. The handle bar has been sticking and takes at least a minute to release once I actually take my hands off of it. What would you say has caused it to stick or jam to the point where it will not crank? This happened rather quickly, as I was using it the day before and it just shut down or jammed up on me.
I acutally took the frame holding the draw string apart from the crank shaft and when I separate the two, the draw string pulls fine, until I attach it back to the crank shaft, then it sticks or jams. The draw string will not pull what so ever. It becomes very tight. Please help, I'm desperate, as my grass is looking scary!
Thank you for any help you can give.