Font change after turning on PC

Maybe it's the font/UI scaling. There is a bug declared in https://bugs.launchpad.net/ubuntu/+source/unity/+bug/1317253

To change the default font there is a similar question Change default system font using terminal only in 14.04

Using "Unity Tweak Tool" you can customize your default font and themes,

Command to install this:

sudo apt-get install unity-tweak-tool gnome-tweak-tool

You may be asked your system password, for installation.

To open Unity Tweak Tool, open the Dash and search for Unity, open the Unity - > Appearance section and select your customized themes and fonts. Check they work after a restart.