As you have not mentioned whether you have a Desktop or a Laptop computer, I am providing the steps for a Desktop machine. Try these steps and check if this fixes the problem:
1. Disconnect the power cable from the computer tower.
2. Press and hold the power button for at least 15 seconds to remove the residual power.
3. Open the computer cover.
4. Reseat (disconnect and reconnect) the Hard Drive connections (both SATA and power connectors) and check if the computer boots. If the computer works fine the issue is fixed, otherwise follow the next step.
5. Connect the Hard Drive to other SATA ports on the motherboard. If the Hard Drive is detected, one of the SATA ports on the motherboard is bad. If the Hard Drive is still not detected follow the next step.
6. Try a different SATA cable and see if the Hard Drive is detected. If the Hard Drive is detected, the problem is with the SATA cable and has to be replaced. Else, follow the next step.
7. If possible, check the Hard Drive by connecting it to another computer. If the Hard Drive works fine in another computer, the motherboard is bad and needs replacement. If the Hard Drive is not detected in another computer also, you need to replace the Hard Drive.
Note: The steps above require you to reseat the components by removing the system cover/taking apart the chassis. Please perform these steps only if you are comfortable else seek a technician's help. Also, refer to the Service Manual for accurate steps and safety instructions.
