If it went away, it went away. If you had an extremely long list of addressees, you should have first entered it into Notepad and then done SelectAll / Copy / Paste so that you wouldn't lose it. You are out of luck. Sorry.
You may have scrolled it off the page. I'm not sure what a "new" From address means. An inbound message has a fixed From address. When you send a new message, you leave the From field alone unless you're trying to pull a fast one; the thing to do is to add a Reply-To field that differs from the From field. You should be able to click on the label (To, Cc, From, whatever) underneath the last such line that you filled in, and it should prompt you (dropdown) to choose "From" or "Reply-To."
I just told you what to do. An e-mail cannot be FROM two different people. It doesn't work that way. Use REPLY-TO as instructed. Kindly do not ask questions and then utterly and completely ignore what the expert has to offer. Thank you.
Create a brand-new message and copy the text from the old one. That will be easiest for you. Don't type in fields when you don't understand what they mean. Fill out TO, SUBJECT, and MESSAGE. PERIOD.
I don't think you have the capacity to interact with the expert or, frankly, even to clearly state the problem.
Would you like me to remotely connect to your machine and try to solve this problem ?
I will do this(remote screen sharing) for just $5 . Will that be okay for you ?