My Mac crashed all of a sudden and is refusing to start .. Late 2011 MacPro 13 " , had just installed Yosemite .. please help ! !

Hello !

I have late 2011 Mac Pro 13 " . Recently updated the OS to Yosemite . Everything was fine , though a little slow .

The Mac crashed all of a sudden when i inserted a usb phone cable in the port .

Has refused to restart since then .

Please help !

Have tried all options such as control command and start etc ..nothing seems to work .

Cheers

Bong Jovi