When you say the address was "invalid", what exactly do you mean? Did you try to make a transaction to it an Electrum rejected the address as being invalid? Or did you manage to make a transaction succesfully, but the receiving address is no longer showing up in your wallet?

Do you mean the address you initially copied is no longer showing up in your wallet? This could be a simple case of your wallet generating a new receiving address as hosseinimr93 has said, or this could be clipboard malware altering the address you initially copied.