Without knowing what "lost" means, I'll assume that means it won't boot. And I'll also assume you are using Windows. And that your hardware is good and you
probably just have some corruption on your HDD. I know - lots of assumptions.
If the above assumptions are correct, I would burn a CD (or DVD) with "System Rescue CD" on it. And boot your "lost" computer from that CD. This may be all it takes to give you access to your hard drive. However, if the harddrive is corrupted, the first step is not to write anything further to that harddrive! You can copy the defective HDD to another HDD using "ddrescue" (that program is on the System rescue CD). Then you can atempt to recover files from the
copy using a program named "photorec" (also found on System Rescue CD). Even though the name is "photorec", it recovers more than just photos.
However, I'm betting you will be able to boot off of System Rescue Cd and be able to access your HDD just fine without having to resort to "ddrescue" or "photorec". If your "lost" computer is a hardware failure, simply remove the HDD and install it in a different computer with good, bootable, hardware.
http://www.system-rescue-cd.org/