Check at BIOS if it's recognized first, then as Area51reopened said, ensure it's in boot sequence.
If ur familiar with hardware, check underneath the hood.
Check HDD power & data cables are plugged in to the system board.
Try switching DVD's ports & cables with the HDD to rule out a possible MB issue (either port or SATA controller issues in Chipset).
& if u have another PC, swap HDDs, that's always a reliable way to isolate hardware problems.
See if that helps, let know how u make out