Installing Ubuntu [and most other Linux distros] isn't too difficult. In fact with the latest liveCD installs it has become easier than ever before. However, it is still possible for the installation process will go wrong and fail. This has happened to me on a couple of occasions and it has left me with a system that is incapable of booting.
The first time this happened there was a power outage mid-installation. On the second occasion the installation shut down after finding a hardware fault [the heatsink had pinged off my coprocessor!] Read about how I fixed my system here.