Just to expand on what has "most likely" happened. Your ignition timing has somehow moved and advanced itself, hence when you try to start it,the engine fires and runs backwards. Has the bike been kicking back lately?
Why it has moved... if you haven't been working at it yourself, the most likely cause would be the woodruff key shearing (as stated above) and the flywheel moving on the crankshaft. If its that, its a relatively easy and cheap fix, though you will need a flywheel puller.