Access ubuntu terminal/sudo after pressing ctrl c and restaring while upgrading system

Question

I tried to upgrade my system from 14.10 to 15.04 and while I was installing it asked me if I want to replace config file. I tried to compare, then didn't know how to exit Ipressed ctrl c and then I tried to restart the computer but I get from ubuntu the message end Kernel panic - not syncing. I thought using the solution right here but how can I use sudo now? How I open the terminal? Thanks