Hi,
Because the file is so large, 9gb in size, it is a big target and can easily be overwritten by new data.
I suggest you use Recover My Files (
Data Recovery Software to Undelete Files; Disk recovery; Recover Deleted Files) to run a "Complete Files Search" on the drive letter for the .pst file type and it may recover fragments of the missing file.
These fragments are most likely corrupt and will not be read by Outlook - so you will need to open them with
www.recovermyemail.com