If you have the latest file system, APFS, then you will need to use recovery software.
I cannot vouch for this software. It will try to recover up to 1GB for free. But, I am not sure if this product has a bootable disk. You may need to attach the drive to another MAC to do the scan. Contact customer service.
If you have HFS+ then you might be able to recover with a live linux disk like Mint Cinnamon. I believe the latest version can mount HFS+ as read only allowing you to copy to a USB external drive but I cannot confirm that.
Edited by JohnC_21, 12 June 2018 - 08:14 PM.