The easiest way is to get an old version from a backup and restore the backup. Then redo the work for the Mac version. If you create a new folder in the project manager, you can import the files from the broken project.
If this is not possible: What is the content that is still available to you? Do you have a WebHelp output with the Windows content?
If you have the files still, you can import them into the project, but they will require cleanup. Rick outlined the steps required: October 2004 - Banishing the sorcery. If you have a lot of topics, you can also opt for my paid, automated solution: http://www.helpessentials.com/downloads/robohelp-recovery-scripts/
If you can provide some more information on what files are still available to you, we may be able to come up with other solutions.
There were only about ten pages, so it was just easier to edit the topics manually one-by-one.
The best information I gleaned from this experience is that when updating content, to never go by the TOC file references as these don't respect their parent folders - always perform edits/updates from the Project Files pod!
Small suggestion to Adobe Developers who put out Robohelp:
1. Learn the concept of intuitive design and implement it!
2. Never release a new product without any support/documentation files!
It's absolutely astounding to me that in putting together Robohelp 2015, they were unable to produce any content for...I don't know, maybe a Help File resource explaining what each component is and some best practices for usage...
After all, isn't that one of the fundamental features of Robohelp to begin with?!?!?!
For William, thanks for the link to "Oct 2004 - Banishing the sorcery". With all respect though, referencing someone using RH 2015 to an article that is over a decade old that speaks to an iteration of RH that is over five legacy iterations out of date, and to a paid 3rd party solution seems to be not much of a reference at all...the problem here is the lack of native support for doing something as simple as duplicating a project. I should not have to go into my OS to do this - the PRODUCT should have this functionality. Is the audience that small where Adobe has decided to pretty much polish a turd and call it "new" and the target demographic just accepts this?
If RH were a truly useful solution for Tech Writers, this should have been addressed ages ago...but then again, I should have known better than to expect this from Adobe.