An operating system wasn’t found problem

First I would check if your boot drive is being detected in the bios, you can do that on most computers during startup by pressing the delete key or F12 (Different systems use different keys you can probably google the one for you.)

From there I would try to see if your boot drive is recognized by the system, if it isn't it may be the case that your drive may have failed which will need to be replaced.

If it is recognized you may have to reinstall windows (or any other OS of your choice), you can do this by formatting an empty USB stick (8GB+ recommended) then creating an installation media from https://support.microsoft.com/en-us/wind... (this will result in any data from the drive being wiped)