Usually this message comes up when there is no OS on your hard drive or other bootable device, like a bootable USB device. The computer does not know what to do when you turned it on, meaning that you either have no hard drive with Windows or another operating system connected to the computer, so the computer is asking for you to connect one.
If you have not made any hardware changes, open up your computer and make sure that the hard drive is still securely connected to the power and SATA data cable. If the hard drive is clearly connected, but the computer still asks for a boot device, then it is likely your hard drive has had its boot file corrupted or the hard drive is physically damaged and not working properly. Luckily a boot file...