I have uninstalled Ubuntu 12.04 and installed Ubuntu 13.04 instead. It told me that Ubuntu is installed and that I need to restart my PC. After restarting I got an error message that said:
operating system not found.
How can I fix this?
Try to boot using the live CD and install Boot Repair program:
sudo add-apt-repository ppa:yannubuntu/boot-repair sudo apt-get update sudo apt-get install boot-repair
then launch Boot Repair, use the default settings and follow the instructions (it may ask you to do something manually)