This is caused by one of two things. A boot device is drive with a software that your system can load. IE: Windows on your hard drive. So either Windows is corrupt, or your hard drive is dead. You can try to de a recovery to reinstall Windows to rule out the dead HDD. To do this turn off the computer and then press and hold the ALT button, don't release it during the whole process. Now turn the unit on and start tapping F10 repetitively. If done right you will see a bar that says Windows is loading files. If this doesn't work you likely have a dead HDD and will need to have it replaced.