Your data is safe because the Simple Invoice software has feature to backup and restore.
Go to main menu and select Tools -> Backup.
There are two options to make backup in program: Manual and Automatic.
When Automatic option is selected then program makes backup each time you exit program. This is recomended option. Backup files are arranged in folders named date like eg. 2019-09-01.
You can also make backup manually, then you will have to select destination folder to save backup file.
Restore option is very easy, just clik Select file to restore button and select backup
file to restore. Note that your current data will overwritten and lost.