Bird nest trouble!!!!!!!!!!!!?

HELP ME!! I was out in my fenced in yard when i saw a nest that had fallen out of the tree. i looked around and saw an egg cracked. i looked an inch to the right when i saw a blue egg unharmed. I picked it up and put it in the nest and put the nest on the otherside of the fence. What should i do if thge parents don't claim it? How can i take care of it? Or is it already dead since it was cold when i put it back in the nest. should i go back out and get it and run it under warm water?? WHAT SHOULD I DO?! HELP ME!!