If you have backups, you need to restore from backup.
Since the file was not deleted but truncated, you can not undelete it.
Recently, I found that, at least Outlook 2007, may have a problem with some NAS drives/servers. Pending determining the cause, I have had to move those PSTs to local drives then have a backup routine back them up to the NAS where a traditional backup gets them with the rest of the data.
- Tom