why would the left channel on my stereo die?

It happened to me on my Adcom amplifier. It was a blown fuse. If you unplug the amp, find the fuses and pop them out, hold them up to the light, and try to see if the wire running through them is severed. Inexpensive and easy to fix.

First of all Lee, are you sure it is the receiver and not associated equipment or a loose wire? Swap channels on your program source and swap speakers. If the same channel remains dead then it is your receiver.
Hilde45's suggestion to check fuses is certainly the place to start if the unit has any. Most do not have user replaceable speaker fuses.  It is obviously not the power fuse. Some speakers have fuses but again this is rare. With receivers there are any number of places in the circuitry the unit can fail. You will have to get it to a qualified tech to find the problem.
This is an older receiver and certainly not worth spending more than $100 to fix. It may be time to get a replacement unit. 

Upgrade NOW!!

This is God's way of saying upgrade to integrated.

Thermal cycling (different coefficients or expansion).

I'm thinking it did you a favor. Check fuses and settings but if it's not something simple move on to better gear. 

You can recycle the Sony at your local Best Buy and you can get something else there too.