If you didn't change your Hard Drive, then your computer won't load Windows because Windows installs everything directly for specific devices. For example, if I have 2 computers and take a hard drive from one of them and put it into the other, it won't boot. You need to reinstall Windows to that computer.
If by any chance, this is not the issue,
in the BIOS move around your settings until you find the boot order. It should list connected devices in there somewhere. If the drive isn't showing up anywhere, it either may not be connected correctly, the boot order is looking for something else first or the drive is dead.