How do I retrieve a lost ISBN number?

The answer depends on what you mean by lost.

If you used it previously on a book other than the one that you are currently working on then you can’t re-use it.

If you were given a free one from somewhere such as KDP Print or Lulu or Smashwords, then you can retrieve it by continuing to work on the project in that interface where it was assigned.

If you ordered a block of ISBNs from Bowker (assuming that you live in the United States), then they have an interface where you can see what ISBN numbers you have been assigned.

If you previously assigned that ISBN to the book, then you can check the Books in Print database through Bowker’s search tools, or if it has been previously published and is available for sale then searching Amazon or other bookseller networks will let you retrieve it.

Based on your question, my guess is that you purchased an ISBN from Bowker, and so the easiest way to retrieve the ISBN number is either to search your email for messages from them, or to log in to your account with them.