You've been "orphaned." This isn't uncommon; my original editor left after the publication of my first book but before the publication of the second (and with the third in revisions.) It's nerve-wracking, and it can be a problematic situation, but it doesn't have to be.
You may get someone who is very much in your corner and enthusiastic about your work, or you may get someone who never read a word of your work and only got the job because it was foisted on them. There's just no way to know or influence that, unless you are already very established. But don't freak out. If you work to build a relationship with this new editor, and it turns out you can work well together, everything's going to be just fine. (Who knows? The two of you might mesh even better.) Definitely ask your agent to arrange an exchange of email or a call -- something like that, so that as soon as the new editor is ready, you guys can get to work on this. (Give it a while, though -- if this person just got a ton of new books to edit, free time is going to be short for a while.)
So be proactive about finding out what's going on, but don't panic.
