Help!!!! mid 2012 macbook wont boot

Hello, Anyone willing to help would be greatly appreciated.

I have mid 2012 MacBook , I turn it on one day to realize that it will not boot. When I try to get it to start it constantly shuts down.

I have looked for every resolution . But nothing seems to be working.

• I reset the smc and pram , that did not work
• I tried to 'safe mode' and it wouldn't allow me too.
• I then tried to re-install OS X ( Yosemite) But it wouldn't allow me to do that either.
• tried to open up terminal and It wouldn't.
• I then proceeded to do ' Recovery Partition' , I went to the disk utility to see if it needed any repairing. Well once I verified it (Macintosh HD) said that it needs to be repaired but it wont give me the option to repair the disk. It also wont let me mount it either.
• I tried to use a external hard drive to back up my files if anything but it says that I don't have any permissions. So it wont allow me to back up the HD.

I have exhausted all options. and I don't know whats going on or if their is anything that I can further do. Anyone's help or insight would be extremely helpful.

Thank you !