I was playing around with
unity-tweak-tool and suddenly unity stopped working, tried restarting but it wasn't effective, reinstalled unity and it started to work normally.
After that, I just noticed that
unity-tweak-tool is missing. I tried to install it from Software Center and after that tried to change theme but it wasn't working, once again tried to reinstall unity & tweak-tool but it was not effective.
OS: Ubuntu 16.04 LTS