The most common error is an incorrect boot order in your computer's BIOS. If you are not familiar with the BIOS, ask someone you know to help you or take the computer to a local technician.
Restart your computer and access the BIOS settings by pressing the dedicated key. The key is different depending on your computer model.
Select the Boot tab and set your computer's hard drive first in the boot order list. Then save the settings and restart your computer.
If you need more details on how to access the BIOS or which device corresponds to your hard drive, please provide the make and model of your computer as well as the names of the devices listed in the boot list.