It means your PC is trying to boot Windows either from a device it can't find, or that Windows isn't on the device. That's often a symptom of either a bad hard drive, or a corrupt/missing Windows installation. First thing is to check your BIOS for the boot order to see that hasn't accidentally got changed. In most cases, it should show booting from a CD/DVD drive and/or a USB drive first and second, and your hard drive third - so, if it can't find Windows on the first options, it will boot from the hard drive.
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