Wow. Dunno what to tell you to do next. If Disk Utility doesn't see it, then something happened to SSD. If you have Windows, boot into it and see if Windows version of disk utility can see it and run a first aid like command.
If that doesn't work, I think your drive is hosed. However, boot using your UniBeast thumb drive and get to the Window that has the four choices. In the menu bar choose Utility > Terminal.
- Enter "diskutil list" note the drive (/dev/diskx) for your MX 300;
- Enter "diskutil unmount /dev/disk?" where "?" is the disk number;
- Enter "fsck -y /dev/disk?"
- If fsck makes any repairs, you'll get a message that says "File system was modified." Run fsck again until the message no longer appears. (The first run may uncover additional errors that will require a further run to fix.)
- Once all repairs are made, enter "reboot".
Good luck!