You try to boot into your Boot Camp, and the Windows partition isn’t shown in the Startup Disk selector or the boot menu that appears when holding Option on boot.
Note: this doesn’t apply if Windows disappears from the Startup Disk selector, but still shows up when holding Option on boot. For those cases, see this article: “After installing NTFS for Mac, NTFS partitions don’t show up in Startup Disk menu”.
This can happen if the hybrid MBR has been changed in a way that breaks the Windows boot configuration.
On Macs released before mid-to-late 2013, booting Windows in a Boot Camp configuration relies on a special hybrid MBR record in the beginning of a GPT disk. The Windows partition must be included into the hybrid MBR and marked as bootable (‘active’).
Native OS X tools don’t include the ability to fix hybrid MBR, but it can be done with the open source utility gdisk (http://sourceforge.net/projects/gptfdisk/). See the tutorial on (http://www.rodsbooks.com/gdisk/index.html).
You can also use Hard Disk Manager for Mac to: