Why are my platy fry disapearing and why cant i find their bodies?

Sounds like 1.) Swim Bladder disease. Are you overfeeding them? 2.) Fish Tuberculosis. A very serious fish disease that will eventually will begin to affect all the fish in the community tank. There are cures for both of them, however fish tuberculosis is very hard to cure and a zoonotic disease(transferable from animals to humans). Do you have gravel in your tank? Might they be somewhere at the bottom of your tank hidden in plants or under gravel? I don't have any real answer for you as to were their bodies have gone unless another fish is consuming them. It would be hard to have 27 bodies at the bottom of your tank without you noticing... I'm just not sure.. But you should research the conditions I told you about to save the rest of your tank.