You have a few options:
- TexStudio may have cached history and allows going back before the fateful save and exit
- Data recovery on the drive can find the bits of the previous chapter that were not overwritten by the new data
- Your computer has cloud sync/backup software (such as OneDrive or Google Drive or iCloud) and you were working in a directory that is protected by this
I am not familiar with TexStudio and you mention it is an old version. If file/edit history is cached this will be documented and will be an option or tool within this program.
Data recovery requires several things: YOU have written anything else to the disk since overwriting this file, the OS hasn't written anything else to the disk since, recovery software can find the bits on the disk that previously belonged to the file, and none of the myriad other factors impacting this work against you.
Hopefully you have a cloud backup/sync tool actively running on your system, as this is the method most likely to work best. Going forward, you should definitely have such a tool going forward, if you do not have one currently. If you DO have such a tool running, you will need to look up how to use it to restore to a previous file version.