Welcome, Lillian!
Can you provide more details about your system and PC specs? I'd suggest you
check the connections to the HDD again and make sure you have mounted it properly inside the PC. If you have a spare SATA cable, I'd try using it instead and also check how the HDD would get recognized if you put in another SATA port on the motherboard. If nothing helps, I'd suggest you give
the BIOS reset a try as well and see what happens. This will get your BIOS settings back to factory defaults, so you might need to re-configure your manual changes.
Keep us posted!
SuperSoph_WD