That message means your computer can't find the hard disk where Windows is installed. There's several things to do, starting from the easiest:
It's possible that your computer has been set up to start primarily from USB so unplug any memory sticks and reboot. The same applies to your optical (CD/DVD) drive, remove any optical disks.
It's also possible that the boot order has changed. That's usually because of a failing BIOS battery on the motherboard. Go to the BIOS/UEFI settings and choose the right boot order. Basically the order of different types of devices shouldn't make any difference, so try to find the hard disk boot priority settings and if there's more than one disk choose the one that has Windows installed on it. If it says that disk isn't #1, swap the cable of that disk into connector #1 on the motherboard to ensure it will always stay as the primary boot disk. If you find out the boot disk order has changed, get a new battery and redo the settings.
If you can't find your hard disk in the BIOS, check that the cables are properly seated. Sometimes the SATA cables are very loose.
It could also be a failing power supply, but I doubt the hard disk would be the first device to suffer from lack of power.
At worst it means the main board of your hard disk has failed. Even that can be quite easily fixed by swapping the circuit board from a similar disk so your data can be saved.
