When you hit F12 during boot, what devices are available? If this is a newer computer you may be looking for a Windows Boot Manger. If it is older, possibly a hard drive is being shown which is bootable.
You want to check the bios in the Boot section and make sure the correct device is listed as first priority for boot. If you had made any changes to your drive configuration, they may have gotten out of sequence or for some other reason is not available.