Probably doesn't apply, but if you've got an SNES-mini (the smaller, newer unit), don't try hooking it up with an S-Video cable - it doesn't work.
As for your current situation, I'm going to recommend trying 18 Days' method, if you can. If you have a N64 or a GameCube that is hooked up and working, try unplugging the a/v cables from it and plugging it into your SNES just to make sure it is actually working.