When you've rummaged through the ashes, saving the little you're able to save, you're entitled to sit down and have a good cry.
Then you need to make a decision as quickly as you can: Do you have enough strength and material to rebuild where you stand? Or do you choose to move along and build a new house somewhere else?
I had lost the only copy of a completed manuscript ready to send to my betas. I still had the nearly illegible hand-written first draft. And if that was all I had, I believe I might have abandoned the book--because I was also without a computer for now, relying on one-hour sessions at the library.
Luckily, I did find a saved second draft on an old flash drive. The book had been through many drafts since then, but I decided quickly:
--I had the strength to work from this.
--I'd recover mentally what I could recall and re-imagine wherever I had to.
--I'd work the way I used to work before I owned a computer.
1) I printed out batches of pages from the flash drive. And since computer time was limited, I returned to the style of my youth: using various colors of ink to signal the different drafts of each page. Before typing anything I would do at least six drafts.
2) I would not obsess over total recall. I would begin by accepting that I could never recover exactly the version that I lost. In the end, life willing, I'd produce a better book.
Progress report: I'm now 1-2 weeks from sending my beta readers the first 100 pages of the Recovery Copy.
My spirits are high. And I will keep you posted!