Have you messed with your BIOS or did a recent BIOS update?
If so, your hard drive mode may have been altered.
AHCI and SATA mode in your BIOS, make sure the correct one is selected.
If you aren't sure, try the first mode, if it works, keep it in that mode. If it doesn't work, switch it to the other mode. If it works, keep it in that mode. If neither modes work or a mode works, please affirm in this thread.
This is a random problem solver generated from the unmountable boot volume error.