The first rule of computing - maintain complete backups. Hard drives do fail sometimes.
Unfortunately, you may have fallen victim to this bug in the installer program.
"Most Linux and Ubuntu users don't go for the automatic options provided by the installers and they manually install the new operating system where they want, but there are also quite a few users who have made use of an option called "Erase Ubuntu x and Reinstall," which, unfortunately, will delete the entire hard drive."
There are utilities that you can try to restore the deleted files, but it is not easy or certain.