I did a terrible thing. Let me start from the beginning. My parents have an external HDD and they store our family videos there and I borrowed it from them because I needed those videos. While we were back in my hometown, I copied the most important videos to my laptop and I kept the HDD for a while, I did not give it back to them. Since I have replaced my laptop (I am selling my Acer gamer laptop and I am buying a cheaper Dell second-hand business laptop for practical reasons) I need an external storage to transfer my data. I installed Ubuntu Linux onto my new laptop and the problem was that the system did not access to the external HDD while Windows could (however, Windows sent me a notification that there was a problem with the drive while connecting it to the computer). So? I tried to fix the partitions on the HDD by ntfsfix. Here is what I received in the command line:
sudo ntfsfix /dev/sdb
Mounting volume… NTFS signature is missing.
FAILED
Attempting to correct errors… NTFS signature is missing.
FAILED
Failed to startup volume: Invalid argument
NTFS signature is missing.
Trying the alternate boot sector
The alternate bootsector is usable
Set sector count to 976773166 instead of 976773103
Rewriting the bootsector
The boot sector has been rewritten
ntfs_mst_post_read_fixup_warn: magic: 0xffffffff size: 1024 usa_ofs: 65535 usa_count: 65535: Invalid argument
Record 0 has no FILE magic (0xffffffff)
Failed to load $MFT: Input/output error
Volume is corrupt. You should run chkdsk.
And then boom! Nothing. The hard drive is not accessible at all. I was thinking about fixing it with testdisk but I am too afraid of causing further damage. So I told a professional to try to fix it instead, which will cost a lot of money (providing the data can still be saved). I am an amateur idiot.