Reintalling a "missing" file

[#1]

I downloaded a book for school (On my Mac Book) and I just tried to access it and it says it's "missing". The book is there but the file is missing. How do I get the file back? The specific Adobe application i'm using is, Adobe Digital Editions 4.5

[#2]

I downloaded a book for school (On my Mac Book) and I just tried to access it and it says it's "missing". The book is there but the file is missing. How do I get the file back? The specific Adobe application i'm using is, Adobe Digital Editions 4.5