You could try something like Recuva ==>>
https://www.ccleaner.com/recuva.
I suspect that whatever is preventing the system from booting is what caused your daughters epub to go south - i.e. it was the canary.
Curious : why does your daughter write (code) in XHTML. Most writers prefer to use a word processor. There are many ways to create an EPUB from a word processing format.
BR