i recently added 2 guppies to my tank (with 3 other guppies already init)
next day one had a bit of a split in its tail i put it down to fighting sorting there dominance ladder out no more of it has dissapeared so maybe its the same with yours?
It should heal back in time, I believe it can be due to a number of causes. However most likely in my eyes is its caught its delicate tail on an ornament in your tank.