Boot device not found
means that it can't find your usual hard drive C: to boot into and start windows.
It may be as simple as you have left a music CD or DVD in the Optical drive and laptop is trying to boot from it. If you have a dvd optical drive that is the first thing to check.
If not then it means the hard drive has either a loose connection or other problem. If you can look in the BIOS settings look to see if the hard drive is listed/recognised. If it isn't, try taking out the hard drive and then putting it back in. There will probably be a screw to undo before you can remove the hard drive. What is the make and model of the hard drive?