Luckily, the following tutorial shows what attempts should be done at first. Therefore, you need to do a lot of attempts. With knowing the exact cause for this error, you could take the corresponding measure, but it's not easy to find the exact cause from all possible causes.
How to Quickly Fix "Reboot and Select Proper Boot Device" Error Windows 10/8/7 If you find one, please share it in Comment part so as to help more people. Of course, there might be other causes for this error. These are all possible causes we found, which could guide us to fix the issue.