Off the top of my head I think it's squawk 7600 and flash your landing light at the tower. I had a complete electrical failure earlier on this year. My first thought was to squawk 7600...which being as it was a complete failure wouldn't have been much use...
. Some fiddling with the master switch got everything back on line although it's a bit of a surprise when you try and talk to your oppo in the other seat and the com doesn't work either. Shouting at each other was the solution.