I've started an entry but don't know how to get back into it!
To get back into your entry, you'll go the Submittable website and scroll down to the category you were working on. You'll see that the information you've already entered but not submitted is still there for you to complete.
Once you've submitted that entry you can start on another, but you won't be able to work on two at once. (Gather all your information elsewhere so that it's ready for your next entry if you have one.)