What kind of screen do you get? Is it a text login screen, where you can enter the user and after that the password? In that case, you can probably uninstall powertop. The first assumption is that the package name is the same, powertop. So try
sudo apt-get purge powertop
If it is not the 'text login screen', you can another menu entry at the grub menu: Select 'advanced options' and then 'recovery mode', when you can get a root prompt and run
(because you are already root alias superuser).
apt-get purge powertop
If you see no grub menu, you can press the left shift key (upper-case key) at once when booting, and it should appear after a few seconds.