Help fixing system that won't boot

[Linda]

I have 12.04 LTS installed on an older machine. It totally froze today and I had to try turning it off and back on. Unfortunately it fails to boot. After alot of searching I found a copy of 12.04 LTS that will fit on a 700 MB CD so I can know boot from the CD and view my filesystem.  How do I go about fixing it so it can boot. When I tried looking all I found was what to do if you goofed up a dual boot machine but this one only has ubuntu 12.04 on it. In help sending me in the right direction to fix this would be greatly appreciated.
                   Thanks
                   Linda