How to change default scaling governor back to ondemand

You can Manually set the governor by running the cpufreq-set command (as root).

Just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the commands below.

sudo cpufreq-set -c 0 -g ondemand

To install cpufreq-set:

sudo aptitude install cpufrequtils

cpufrequtils includes a daemon which allows users to set the desired scaling, governor and min/max clock speeds for all processor cores at boot-time. Before starting the daemon, edit /etc/default/cpufrequtils as root, selecting the desired governor and setting the min/max speed for your CPU(s), for example:

/etc/conf.d/cpufreq

#configuration for cpufreq control
# valid governors:
#  ondemand, performance, powersave,
#  conservative, userspace
governor="ondemand"
# valid suffixes: Hz, kHz (default), MHz, GHz
min_freq="1GHz"
max_freq="2GHz"

Source: archlinux.org

You can use following command to change governor:

cpufreq-selector -g ondemand

With this command, you don't need root privileges to change governor. Just add this command to Startup Applications... to change governor on login.