One of my key on my lenovo y40 came out earlier today and I have no idea how to put it back in?

If just the plastic piece with the letter/number stamped on it came off, simply sit it back in the space and press down on it and it should snap back in… If it doesn't then you may have broken something and need to buy a new key… I'm having trouble finding keyboard keys sold individually for your particular make/model but I know they sell them… You might have to call a local computer store or something and see if they can order it…