No power to switch and outlet after replacement attempt
I have a stacked light switch that controls a back porch light and some landscape lights. They are on different circuits. I wanted to find a new switch that would allow me to dim the back porch light on occasion. Thought I found a Lutron switch that would allow me to do this, but it only had three locations for wires, while I had four (two black, two white) to find homes for. I asked an electrician and he thought I could use one black wire to power both switches and then cap the other. (Perhaps he didn't understand my situation - I'm not sure.) I came home and tried that and my first attempt caused both circuit breakers to trip. I wasn't confident trying much more so I disconnected the new switch and went back to the old switch, wiring it just like it had been.
Now, my outlet (GFCI) for the landscape lights isn't working. I tested the GFCI, which had not tripped, but it will not reset. I tried a replacement GFCI and it also will not reset. Thus, I'm guessing the outlet isn't getting power. The porch light works fine. A second outlet on the landscape light circuit works fine as well. I pulled the outlet box for the stacked light switch and everything looks good back there. So, I can't figure out what went wrong. Any ideas out there?
Thanks so much,