Internal Hard Drive Not Detected after El Capitan Update

I updated to El Capitan: It was working fine (i guess?) was using Safari, it lagged and the rainbow wheel showed up (its stucked) thus i restarted my laptop. then next thing i saw is a flashing "?" folder and it ended up unbootable.

Things i tried:

1. Restarting and holding Option/Alt Key - DIDN'T WORK

2. Resetting the NVRAM (Holding Command (⌘), Option, P, and R) - DIDN'T WORK

3. Target Disk Mode (Holding T) - DIDN'T WORK (bought a new thunderbolt cable and tried connecting to 3 different MacBooks, all 3 MacBooks were unable to detect my macbook)

4. OS X Recovery Mode (Holding the Command (⌘) and R): DIDN'T WORK

4.1. Disk Utility > My Startup disk doesn't show thus i can't do a FIRST AID nor a RESTORE or even an ERASE.

4.2. Reinstalling OS X > No Startup disk so there's no way to proceed.

5. Booting OS X Yosemite from an external Hard Disk: DIDN'T WORK

5.1. Using the Command Line/Terminal - diskutil list (doesn't show my internal hard drive)

6. Using TestDisk to recover and fix it but: DIDN'T WORK

6.1. Again the Command Line/Terminal can't detect the original startup disk (internal HD) - diskutil list (doesn't show my internal hard drive) thus nothing to repair/recover.

Is there any other way I can fix this? I went to an authorize Apple Service provider this morning but they told me it will take minimum 20days for them to be able to fix my laptop as there is a queue (don't want to bring it to other place as it is still under warranty - my laptop is 6mos old and i still got 6mos remaining on my warranty plus repairs are quite costly here). I think 20days is too long as I'll be traveling soon.

Thanks for your help! 😀