Attempted to change permissions on external hard drive files via command line, now several terabytes of data appear to be missing

I was attempting to modify the permission of my 4TB external hard drive to enable streaming to another machine via a third party app. The process was taking a long time, so I left the computer alone and came back some time later. On return, the computer had gone to sleep, and when reexamining the drive, I saw that the filesystem appeared empty, whereas before there were 2.3TB of data. On viewing the Properties menu of the disk, it said that there were 5GB of data.

Is it possible that the original data is still present, just hidden somehow? If so, how could I recover it?

I’m relatively new to ubuntu and would appreciate any help, as many of the photos on that hard drive are irreplaceable.