Small Engines (Lawn Mowers, etc.): Murray I/C Quiet riding mower, crankshaft pulley, engine crankshaft

Question
While mowing the grass this evening the blade belt came loose.  It is not broken, but it is not in the right position anymore.  Can you direct me to a diagram that shows the belt configuration and give me instructions on putting back in the right position.  Or...do I need to buy a new one.  Why would it come loose like that, anyway?

Answer
Is there an idler or blade spindle bearing bad?  Is the main engine crankshaft pulley tight?  Are the engine mounting bolts tight?  Are all the deck hanger brackets in good condition?

The mower model is located on a sticker, typically on the rear of the mower under the seat or rear fenders.  The sticker is about the size of a business card.

Please send the mower model number so I can see if a belt diagram is available.

Thanks.
Eric