The goop didn't come from the optocoupler. It's hard to tell from the picture, but is one of the capacitors on the left bulging or leaking? Is the goop wet or does it feel dry and rubbery? It looks to me like it might be silicone adhesive applied when the thing was built.

If you heard a faint pop, check to see if the line fuse is blown. It's not uncommon for the chopper transistor to fail shorted, usually when a solder joint gets flaky. If that happens it will blow the fuse. You could replace the power supply easily enough, but they're usually pretty easy to repair.