I'm so sorry, this happened to me too a few months ago with my 30g community tank and it was really devastating. One day, I noticed one of my black mollies had died, so I removed him, did some tank maintenance and that was that. Then another fish died, so I became worried..I did the same thing. Then the next day, all of my fish except a Giant Danio and a red Swordtail were dead..It was really depressing because there was absolutely nothing outwardly wrong with them, and I didn't know what was wrong or how to treat it.
I just wanted to give up, but I didn't...These things can happen and unfortunately the only thing we can do is take extra care and watch closely to make sure we can catch something going wrong before the worst happens. :)