Yes you can reinstall, and with luck get back to where you were. I have to say "with luck", as it has to depend on exactly what's gone wrong. If you have backups (you do don't you???) - then you won't have a problem anyway.
Otherwise the only 100% safe way to not lose anything is to copy the whole drive to another one, before you start - time consuming but as safe as you can get.
Anyway - to re.install - boot up from the CD, and contnue just as if you were doing a first time install, until you get to the point where it say it's detected a prior installation - do you want to do a fresh install (NO YOU DON'T) or a repair. Choose repair and go ahead.
I've done it a number of times successfully - though once it fairly screwed things up.
seeing s you cannot boot the machine at all ,before you wipe out anything make sure its not a hardware issue like a loose wire or memory module, youll be really annoyed if you re-instal then find it still happens.
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