Edit /etc/passwd file from grub

I tried installing zsh on ubuntu ( 14.04). When I switched to zsh, I was unable to go into super user mode.
If I do a su it asks for passwd and on entering the correct password, it says: Cannot execute which zsh: No such file or directory But when I enter an incorrect password and press enter 4-5 times, it shows nothing. On checking my /etc/passwd file, it showed: root:x:0:0:root:/root:which zsh I tried to edit the file and save but it shows Unable to save /etc/passwd I have tried to change the runlevel to 1 via grub, but it results in an error: APCI PCC probe failed I am running Ubuntu on VirtualBox.