There's a few potential causes...and potential solutions.
1. Is your HDD/SSD detected in the BIOS? If so, you may need to 'repair' your OS. If it's not, you can try another sata/power cable and see if that lets it be shown in the BIOS. If it does not, your HDD/SSD may be damaged. You might be able to repair, you might not.
2. If the HDD/SSD is detected, is your OS drive set as boot priority #1? If not, set to first boot. If it is.....you might have a bigger issue.
3. f you HDD/SSD is showing in BIOS & set to boot priority #1, you'll likely need to 'repair' your OS. You'll need bootable media - either a USB or Disc. Following the prompts (as if you were doing a clean install) and select 'repair'.