It would be nice to a screenshot of the password dialog even if you need to take it with a camera, so we can see what kind of a lock it is. It would also be useful to know the exact model of your computer, and that of the hard disk if you can find it. And also if the drive in question is the system drive or not.
This is important, because if it is really the hard disk that is locked, then there is no simple way of unlocking it. Normally, its firmware will even refuse to reformat the disk without the password. This can happen because someone locked the drive in the BIOS, or because of a hardware glitch.
The best option for you in this case is to get in touch, first with with the Support for your computer manufacturer, then (if that doesn't help) with that of your hard disk. You will need a proof-of-ownership of your computer to have a hope of getting an answer.
As far as software is concerned, the article Unlocking a password protected harddisk claims that the atapwd program can do the job. The links no longer work, so this and other such tools may be found on this Hard Drive Tools page. However, be warned that using these tools is quite risky to your hard-disk and computer and that I do not have any experience with any of them.
A commercial service may be found at Repair Station, and again I have no experience with them.
There is a person that claims unlocking his drive using two hard disks. What he did was set a password in the BIOS on his unlocked disk (meaning lock it), reboot and go into the BIOS to clear the password. The BIOS will ask for the password to be typed in, so enter the password from above, and when it then asks for a new password replace physically the original hard disk with the locked one while the computer is still turned on and then enter an empty password, followed by as many OKs as necessary.
I have no idea what will be the result of the above procedure and take no responsibility.
Here is how I personally solved this, without any claim of deep knowledge or understanding.
First disable the lock / HDD password protection, as described here, Reset HDD user password, if I know the HDD master password?, which I have from https://serverfault.com/questions/712849/how-to-unlock-a-ssd-disk-with-hdparm/733784#733784
This solution requires that you know at least the master password for the drive, and that you have a running Linux OS with the disk connected.
I think the main conclusion is: BIOS vendors do what they want.
One password, multiple HDDs
My observations indicate that when I did specify a HDD password in BIOS (with the BIOS version that I am using), it did affects all HDDs that were connected at the time: Both the onboard SSD, and the 2.5'' SSD.
A friend told me that his BIOS has separate settings per HDD. Which makes a lot more sense.
My personal conclusion from this is that I won't use the hardware disk lock (or hardware disk encryption), if the BIOS is unable to have separate settings per disk.
what then happens if I change the password while only one of these drives is still inserted, and the other drive has been removed?
E.g. if one drive is password-locked, and the other is not. Or both are, but with different password?
I imagine that my particular BIOS will simply not handle this very well. This is why I will not use this feature.
User password vs master password
As said in Reset HDD user password, if I know the HDD master password?: I was able to unset the user password using only the master password, with hdparm. All data became accessible.
However, in my BIOS this seems not possible.
The following article, https://www.zeitgeist.se/2014/09/07/enabling-ata-security-on-a-self-encrypting-ssd/, suggests that there is actually a setting, "MASTER PASSWORED CAPABILITY BIT", which defines whether the master password can be used for unlock and for disabling security.
The article also mentions that BIOS is an unreliable tool.
lsblk gives a good overview of disks currently in the system.
hdparm -I /dev/sdx tells us whether a drive is locked, frozen, and other things.
hdparm can be used to unlock a drive and disable the security.
In https://www.zeitgeist.se/2014/09/07/enabling-ata-security-on-a-self-encrypting-ssd/, it is suggested one should use
hdparm for everything, and not trust the BIOS. I am still reading up on this.
real disk encryption:
I am still not sure whether the password I defined in BIOS actually did enable the hardware-level disk encryption.
Well technically it is always enabled. But will the password be used to decrypt the disk, or is this something separate? I might find out more.
In an answer to a similar question I found a link to an article on HDD passwords. The HDD password is defined as part of the ATA specifications, and it's implemented in the drive firmware. This is a simple lock, but since it's implemented in the drive, it can't be bypassed by clearing CMOS or connecting the drive to another system. Some technical knowledge (and possibly some additional hardware) is required.
It's not possible to recover the password (at least not without disassembling the drive controller chip), but getting around it could be accomplished with additional hardware. If it's your laptop's boot drive, you'll need to boot to CD/USB or connect the drive to another system.