That wiring looks decidedly dodgy.
There is short protection and overtemp protection as far as I know, however these things are not indestructable, if the amp went into oscillation then temp rise can be very rapid.
There might be a stabilization network on the board, connected to the speaker output, a cap and resistor, if these parts show signs of being hot then thats the cause.
Could be either in your case, you've definately not had ground connected and most likely a short, if you sort these 2 issues out chances are it'll be fine with a new chip of course.