Operating system not found

@barakab you are really not giving us a lot of information here ;-) The most common reason for that is typically caused by an issue with the hard drive or the boot order in the BIOS.

To start fixing this, you will need to use a bootable USB or DVD. Something like a Linux Lite bootable USB is best since you can use that OS to check the access to your HDD. If the bootable OS does not see/find or can't access the HDD, it has failed. You will need to replace the boot drive (HDD I assume)

If the bootable OS "sees" it and can access it then it is most likely that you are having issues with the boot order or the boot sector of your installed OS (Windows I assume). In that case reinstalling your OS will resolve this. Let u know what you find out so we can guide you further.