PLEASE HELP! I mounted the new HDD to existing folder (where another HDD has been mounted), and all files (in other HDD) have disappeared [closed] - linux

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I have recently bought a new 8TB HDD (barracuda) and tried to connect and mount the HDD to ubuntu system.
However, I made some mistakes and accidentally mounted the HDD to existing folder, where another HDD has been mounted.
Specifically,
(Before buying new HDD) I had 1 SSD & 1 HDD (will denote as HDD1 from now on), where
SSD: /dev/sda ==> Mounted on /home/{username}/SSD via
mount /dev/sda /home/{username}/SSD
HDD1: /dev/sdc1 ==> Mounted on /home/{username}/HDD1 via
mount /dev/sdc1 /home/{username}/HDD1
After buying new HDD, I have connected the HDD and
tried to do:
HDD2: /dev/sdb1 ==> Mount on /home/{username}/HDD2
what I have actually done
mount /dev/sdb1 /home/{username}
--> Mounted /dev/sdb1 to /home/{username}
After running this comman, all the files in HDD1 and SSD has been removed, and has been overwritten by HDD2's files.
I have read the post where the files are shadowed if mounted over it, but I am keep getting trouble restoring these files, and can't even find the files via
du -sh *
or
df -h
at root.
Are there any chances of this procedure overwriting the files in HDD1 and SSD? Are there any ways to restore back the files? PLEASE HELP!!!!!
WHAT I HAVE TRIED
1.
sudo -i
sudo umount /home/{username}
but
error: target is busy
came up.
So I have killed all the processes running on /home/{username} via
fuser -ck /home/{username}
Now I am completely locked in a state where I can't open /home/{username}/HDD and only terminal is available.
2.
I have tried df -h after doing (at root user, via sudo -i)
and following messages come up.
df: /home/{username}/SSD: Input/output error
df: /home/{username}/HDD1: Input/output error
Filesystem Size Used Avail Use% Mounted on
udev 63G 0 13G 0% /dev
...
/dev/loop14 56M 56M 0 100% /snap/core18/2697
/dev/sdb1 7.3T 3.5T 3.5T 51% /home/{username}