Zoom Webinar recorded only one screen, not the one who was presenting

Welcome to the Zoom Community, @HelenaCP.

I'm assuming you are talking about Cloud recording.

If the video you're looking for is not in one of the video files you've downloaded, there's no way to "go back and restart the recording".

It's an extremely rare occurrence that a recording will "spontaneously stop" without human action.  I can't say it "never happens", but so rare, and still unrecoverable.

It is possible someone stopped the recording and, realizing their error, re-started the recording.  Note that in this case, Zoom actually creates two separate recording files, which are sometimes difficult to locate the first time you run into this situation.  (Zoom only creates one recording if you hit Pause; but stopping and restarting always starts a new recording.)

The thumbnail on your recordings web page looks like this:

Here's what you see when you click on the recording thumbnail when there are two recordings:

You might look to see if there is a second recording down below the first.  If there isn't one, then sorry, it's unlikely you'll be able to get the second person's presentation.