What happened to my Guppy!??! HELP!!!!!!!!!!!!!!!!!!!!!!!!!!…

My female guppy was in perfect condition, swimming eating etc.etc. However, about an hour ago, I noticed that my once pale/blue tailed looking guppy female is now completely black, the body of my poor guppy has turned black like coal. Furthermore, she is swimming in a wilting, barely shifting way and is not active as used to be. All the other guppies look fine, swimming happily w/ the others and the water conditions seem ok. Please tell me there is a way to save her, I have grown attached to them, if she is to die, dont tell, just give solutions. I would like to know if this has happened to previous guppy owners. I think she may have been injured by the filter, but why would the whole body turn black? so I am asking for help at the middle of the night.pls help.