I have changed the permission of The root folder, where /tmp and /usr are to:
sudo chown - R root *
Now I cannot login, only black screen is showing and everything is a mess. Is there anyway to reverse it to normal. Should I do a fresh install?
You have accidentaly changed the owner of every folder to be root , thus you are unable to login anymore.
Most probably you havent set the root password , if you have just skip this section:
(recovery)at the end of it.
Drop to root shell promt
mount -rw -o remount /to get rid of the read only thing
passwd, which will prompt you the enter a new password for the root
Here you have successfully set up a new root password , if you are having problems then refer to this question.
Part 2 Here you are going to change the permissions for your home directory.
Ctrl + Alt + F1
chown -R root:username /home/username
chown -R <username>:<username> /home/<username>. Change the username to your username.
Hope it helps , thank you.