What happened to my fish?

[Ali]

Background: I have 2 fish, a sword tail and a guppy. I have 2 tanks, one is the tank they're usually in and the other is the tank I use if I need to quarantine one or need them out of the other tank for any other reason. I've owned both fish for about 5 months now. Well today when I woke up I went to feed my fish as usual. But when I walked up to the tank I noticed something strange: the sword tail was just fine, doing his usual thing, but my guppy was on his side, jerking frantically and breathing very heavily. At first I thought he had somehow gotten stuck in the gravel, so I very gently tried to get him unstuck. He did begin to swim around the tank a little but he was still sideways. So I put a little pond salt in the quarantine tank and put him in there gently. Well it's been probably an hour, and he's not moving. I'm absolutely sure he's dead, but I want to know what could have possibly happened to him and if I should be worried about the sword tail.