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I'm afraid that this error usually indicates a failed hard drive.
It sounds like you may already know this, but when you first turn your computer on, a program that resides on a chip on the motherboard called the BIOS searches for a boot device of some kind to hand control of the machine to. In your case, it's looking for a functional hard drive with Windows on it, but it's not finding anything and giving the error "boot device not found."
The first thing you'll want to do is disconnect or remove any USB drives that might be connected, including external hard drives and flash drives. You will also want to remove any discs from the disc drive, if you have one. Then reboot the machine.
If that does not resolve the issue, or you do not have any external drives connected, then I'm afraid it looks like you simply have a bad hard drive. It's also possible that it could be a bad hard drive cable or a bad controller on the motherboard. But what you will need to do is take the machine to a local shop and have someone do a hands-on diagnosis of the computer to track down the problem.
I know this is frustrating and I really wish I had better news for you! I have had to deal with this problem myself in the past. If you have any other questions on this, just let me know.
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