A bit of potentially relevant background: Ubuntu 18.04LTS has not had exFAT support until quite recently (due to exFAT being proprietary and not fully supported in the mainline kernel until 5.4) and even in recent BitCurator releases (2.2.12 and up) that are based on Ubuntu 18.04.5 with the 5.4 kernel, it may be required to install the exfat-fuse and exfat-utils before exFAT drives can be read. I'd need to test to be sure.
That said: if you plugged in an exFAT drive via USB *without* updating the kernel or installing these packages, and attempted to mount it, Ubuntu would typically pop up a dialog that said "Unable to access USB" followed by some addition information about the specific device, and would not allow you to read anything from it. Since it appears you were able to mount the physical drive, it's possible that the drive was incorrectly formatted to begin with, or was somehow recognized as a regular FAT drive. It's not easy to say without additional info on the specific version of Ubuntu (BitCurator) that was being used, whether any updates had been applied to it, and how the drive was physically plugged in and mounted.
Acknowledging that data recovery procedures were *not* the question you posed here, it's unlikely that your data is actually lost (overwritten), but rather that the information that describes the file system on the disk is in an inconsistent state or damaged. A potential recovery procedure would be to make a sector-by-sector (raw) copy of the drive to an identical 16TB drive, and run an exFAT file system repair (or in the worst case, a file carving tool) on that.
Kam