As long as the file has actually been recorded and properly saved (with audible ausio and not only silence), it should still be present on your hard drive. I doubt that Reaper can erase files accidently while crashing. Reaper is completely non-destructive. If you had not created a project and saved that into its own exclusive folder prior to start recording, Reaper records everything into its temp folder (specified in the Reaper preferences under GENERAL -> PATHS ...). It's highly recommended to first save a project into its own project folder with its own individual name BEFORE recording any audio. Also, it's recommended to specify a relative folder for the project media in the project preferences (FILE -> Project settings -> Media). This ensures that everything you record gets saved in the respective subfolder of your individual project folder. To find the missing file, you should try "Everything" (ultra-fast search-as-you-type search program for Windows, freeware, portable). Do you remember parts of the file name? Then type this into Everything. You can type certain strings to narrow down search results like adding "wav" (without quotes) or wildcards "*", "?" and sort the results by date. Unfortunately, reapeak files cannot be converted into wave files. If a conversion would work 100 %, then wave files would be obsolete and full resolution audio files could be stored as peak files by default, being 10 times smaller than lossy mp3 files. It would still be interesting, what audio quality would result in a peak file conversion to wave as the peak file can show every single sample of the wave file it represents.

I agree with Everything search...

Tried and... 

Downloaded Everything, searched by file name and partial. Only brought up the saved .rpp file which is empty.

That's bad/sad news. To fully understand the situation: You were rendering the a track to mp3 format via Reaper rendering window when Reaper crashed? You are saying that the wave source file is still there but it is empty (only silence)? Did you ever play it back after recording it (before rendering)? My guess is that something went wrong already during the recoding process of the source track (file got corrupted somehow) so that Reaper crashed when trying to render it. Hope I'm wrong!

Paging Dr. L to the emergency room, bed 1, Dr. L to the emergency room, bed 1. Dr L. "ok whats going on with the patient?" Nurse "sir, its the saved file in reaper, we're losing the file!" the nurse said in frantic voice. Dr L's eyes grew wide. Dr L. "Quick hand me the scalpel!" As the nurse handed the scalpel to Dr. L the scalpel is dropped to the floor. Nurse "Is everything okay Dr L?" Dr L. "I think so, a voice in my head just gave me an idea" Nurse "Well what did the voice say" Dr L. "It said to go onto the reaper forum and ask for help".....

Sounds like a system and or hard-drive issue to me. Does the Mac have an equivalent to FDISK? >

Isn't there a backup project file? Try opening that and it should pull up your media file. Just a thought.