Not seeing my HD in the disk utility

I tried to start up my computer and got a flashing file with a question mark. So I held down the ⌥-key on boot and brought it into recovery-mode.

After going through the different options I went to the disk utility and all I see is the grayed out SuperDrive and below there is disk0 and Mac OS X Base system.

Leaving disk utility I went to reinstall Mac OS X. After the verification and agreeing the terms, it doesn't show my harddrive.

Did I unmount the harddrive or what could the reason it's not showing up?

I am assuming that you got a flashing folder with a question mark similar to this:

That means your Mac cannot find the boot loader.

Now, since you said that you were able to go into disk utility, you already know how to boot into Recovery Mode. What I am assuming you saw when you were working in Disk Utility is something similar to this:

The SuperDrive being greyed out is probably because the drive is empty. disk0 is typically the disk drive in your Mac. Mac OS X Base System is more than likely a partition on disk0

Do this:

Hold down the Option while powering on your Mac. If there is a bootable partition, it will be listed (other than the recovery partition). If so, boot into it.

Once you have booted, check to see what you have selected as your startup disk (Preferences --> Startup Disk)

My Mid-2012 Macbook Pro 13" randomly stopped working due to this issue. I thought my 8 year old OCZ Vertex 4 SSD was the culprit since my 10 year old Vertex 2 died in a Toshiba laptop.

The problem was the internal thin black flexible cable (less than 1 mm thick) that connects the drive to your Mac's motherboard. This cable's copper is fractured and transmitting data incorrectly making your Mac believe that it cannot find the boot HDD or SSD.

The cable is the weakest link since it is sandwiched between your hot HDD/SSD and the rough brushed aluminium macbook.

In the future, place electrical tape on the rough aluminum to reduce abrasion. The electrical tape backing is quite a slippery and smooth rubber surface. Tape is not meant to hold the cable in place, it acts as a comfy bed for the cable.

Apple later made a thicker cable with a diferrent model number to address this issue. Fortunately it is easily purchasable for $10-25 on Amazon (as of 2021) and was pretty easy to install with only a small philips screw driver.

There are 3-4 different cables for 2009, 2010, 2011, 2012 macbooks so get the right one. Cables are NOT interchangeable since they come in entirely different lengths and won't fit right. The cable model number is written in white on your broken cable.