There is a 'Recovery Mode' boot option on startup;
This mode just loads some basic services and drops you into command
line mode. You are then logged in as root (the superuser) and can
repair your system using command line tools.
Unlike a Windows recovery it will not automatically fix any errors - you will need to use command line tools and understand exactly what you are doing to repair your installation.
As Michael K suggested you can also use a live CD to recover an installation but once again you must know what you are doing and use command line tools to carry out the repair. There is a good Ubuntu Documentation page on Live CD Recovery that I recommend you have a read through to get a good understanding of what is required and how to use it.
A third option is to do a re-install of the operating system but do not format and just 'overwrite' your existing installation.
- Boot to the install media and go through the install process as usual.
- When you get to the partitioner to allocate drive space, depending on the version you are installing check the box marked 'Manually select partitions' or 'Something else' and click forward. Use the SAME user name and password as the existing account when setting up again.
- Find your existing installation and you must UNCHECK all the 'format' boxes and mount the partitions in the same place they are now.
As long as no 'format' boxes are checked, no data will be lost in your home folder and the system files will be overwritten with the originals. You should then complete all system updates again.
Regardless of what option you choose to use to try and recover an installation you should regularly back up your important documents and files so that you can restore them if needed.
Read through the answers to Comparison of backup tools for a comprehensive list of recommendations.