Guppy With Hunched Posture

Guppies are often sickly fish nowadays. Selective and intensive breeding from pretty but not very healthy fish is common, resulting in yet more sickly fish. And the conditions they are bred in are not conducive to healthy fish either.
What I'm getting round to saying is that it is quite common for guppies to just die. I've read of so many people who won't keep guppies any more after all the ones they bought died.
Did you buy all your guppies from the same shop at the same time? If you did, it is possible your fish are from a bad batch.