During the installation it mention something about sandbox or something like that. But it continued normally. The installation was interrupted twice by a message that said only 500mb are free, delete some applications, etc. I pressed the option that would have allowed me to do that but never actually opened until the end of the installation were a message announce a mistake that made impossible to finish the process, said the system might be unusable and that a recovery stuff would start but that never happened. I can open some programs like this one and libre office but can not start the software center and the update manager. I've considered downloading the OS from Internet to a CD and reinstall but I know I'll lose all my applications, and documents, I made a backup before so my files are safe but I don't want to install all my apps again. Is there any other way? Thanks for any help or advice you can give me.
You have no free space. That's why your upgrade failed and you are stuck in an "unusable" state. Please delete files to free up some space -- you can start by deleting the files you already have a backup of -- at least 1-1.5GB is recommended.
Once that's done, open Terminal
Sometimes this won't work either because the system thinks you have upgraded to 12.04 but you are, in fact, stuck half-way. You can do this then:
sudo cp /etc/apt/sources.list /etc/apt/sources.list.backup sudo sed -i -e 's/precise/oneiric/g' /etc/apt/sources.list sudo apt-get update sudo do-release-upgrade