@KatherineSteele I am so sorry to hear about this! Unfortunately, items deleted with OmniDiskSweeper are not recoverable. My first line of defense is usually to restore from a Time Machine back up, but I see you don’t have one. Some other options: maybe you emailed the file recently? Did you store it through a cloud-based syncing service such as iCloud or Dropbox? Alternatively, you could look into a data recovery service (OmniDiskSweeper removes the files but does not perform a secure erase).
i’ve used some software called ‘Disk Drill Media Recovery’ - just google it and it should come up - when i was loking for some photographs that i’d accidentally deleted. it worked pretty well - you can even set the file type parameter for the search.