Blinking question mark folder- system drive not recognized

Okay, so I know there's other threads describing this issue - but nothing has helped me thus far. I've got a MacBook Pro 13" Mid 2012 that worked perfectly until about a week ago. Suddenly, when I started it, the blinking question mark folder appeared.

This is what I've done so far (but nothing has helped me):

• Bought a new SATA cable and got rid of the old one
• The system drive (SSD) works with a USB cabinet, also worked fine after formatted in a different computer - however not recognized at all in my system
• Tried with another SSD drive with Mac OS-X installed - not recognized
• Tried to boot with a Mac OS-X Installation USB stick, and a different SSD drive - not recognized in Disk Utility
• Tried all kinds of different commandos - CMD+R, CMD+ALT+P+R etc.
• Tried the system drive in my optical place where another drive's been recognized, but it didn't get recognized

Can't get anything to work.

Has anyone got any idea of what the issue might or what I could try next?