Ironically, my mis-step with the BIOS came about because I was trying to prepare a second HDD to back up the first one, in case of HDD failure !
If you were trying to configure a 2nd HDD in the BIOS, it wouldn't have affected the boot sector / MBR on the existing HDD. Are you sure you haven't just removed / disabled the existing HDD from the BIOS settings? If you changed the CHS settings in the BIOS, so that the drive isn't recognised, then you need to identify what they should be and reset them - the HDD should give some clues, and you can often look up online information, although I'd be surprised if a disk that old has any reference material online. You may be able to work out CHS settings (if that's the problem) from first principles, given the size of the disk. If you are lucky, there might be an Auto option for the disk config in the BIOS, but it might be a bit old for that.
If you get the HDD caddy I'd suggest you make an image backup of the disk before you do anything else.
If the OP follows the advice to put the old HDD in a USB caddy and copy the data from it before doing anything else, why would it be necessary to create an image of the disk? If a file copy is successful, then what benefit would a block copy be? If the data can't be copied off as files, and other tools / options need to be used to try to extract data, then yes, a block copy to a blank disk will allow multiple data retrieval attempts. But since a file copy doesn't involve writing to the original disk, it's a zero risk first approach.