mac crashed while installing mountain lion - cable disconnect I downloaded mountain lion on my mbp which is run by snow leopard atm. I started to install it and waited for the installation to end. When it came down to the last 12-11 minutes, I ended my distraction and pulled the computer a bit in my direction to keep track of the status bar. With this movement I managed to rip off the charging cable! (I mean, I completely broke it) The monitor turned black instantly and the computer was shut down. When I started it again, the installation wizard shows up again and asks me to press the continue button. However it shows no reaction to any of the buttons available and thus I cannot continue installing. 

I thought ok, lets install the old snow leopard again and try booting from CD, but by doing that, the good old mac os greyscreen appears after a few seconds. 

That means: 
I can not continue installing and I can not install snow leopard again either. I am simply not able to finish booting at all.

Any suggestions how to proceed?
Try to hold down command+R, this should boot the Mac from your recovery partition. You should be able to reinstall from there.