An operating system wasn't found. Try disconnecting any drivers that don't contain an operating system. Press Ctrl+Alt+Del to restart and over and over what shall I do

You can go into the BIOS settings to see if the hard drive is shown there, if it is not then it is toast. You can also take it out and use another computer with a converter to test it for other issues. Or you can just replace the hard drive.

If you tried cleaning your CPU some wires could be loose so try removing and putting them again or sometimes if you have a USB plugged in while turning it would be the reason so remove the USB and turn on the desktop again

HDD has failed or there is USB drive plugged in and your bios has the first boot device as USB, remove the USB drive.

You have a virus and it’s wiped the boot record, do you have a DVD disk on the machine and a boot disk you can reload the operating system.

If not borrow a friends laptop download Win 10 from the internet and burn it onto a Dvd or usb disk using this https://www.microsoft.com/en-gb/download/windows-usb-dvd-download-tool or https://www.lifewire.com/how-to-burn-an-iso-file-to-a-usb-drive-2619270

If it’s a modern machine then the Bios will hold the license for MS - then just sync your cloud to retrieve your data, assuming you’ve backed it up.

PS buy an SSD drive from Amazon they are quiet cheap now and 256 Gig will be plenty.

Have you tried going into Safe Mode or System Restore?

What can i do if my computer show me . AN OPERATING IS NOT FOUND AND TO DESCONECT THE DEVICES