what will happen if i resend money to exchange site address will gone or back to user ??
If you're sending money to an exchange address, the older address should likely work. Don't take my word for it though, search your exchange up.
If you're referring to the address that sent you the withdrawal then, It will be gone most likely but If you're sending it to the deposit address then the user should receive it, as long as the exchange doesn't generate a new address each time and makes the old ones invalid.
this is my ask what will happen ?? to who will go ?? or just will reduce number of bitcoins
Any Bitcoin addresses generated will remain valid forever. If the exchange "invalidates" the address, they are just implying that they have stopped recording deposits to that address. As long as they have the private key, your Bitcoins should still be safe.