When you installed the OS could you tell us if you performed a fresh install, an Archive and Install or an upgrade?
Let us know
We usually deal with a few people at a time so if it takes a couple of minutes for me to reply dont wory.
Now, Can you tell me what you know to be missing, Documents, Programs?
Could you open your Mac HDD and see if you have a folder called Previous System which should be located in the /Previous Systems/Previous System folder
If you do could you open it up and let us know if all of your files are there so we can recover everything.
Could you try to search the drive using your finder search box for anything called previous system.
If it does not find anything let me know.
I am very sorry to say but it sounds like you have overwritten your previous data, However there are a couple of alternatives in this situation.
The first is to use some form of undelete or format recovery software like Macintosh data recovery http://www.macintosh-data-recovery.com/ or Kernel http://www.datarecoverymac.com/ A free trial can be found at both of the links I provided.
If you download the trial version of the software to see if it can find your files you can then purchase the full version to recover your files for $129.00 or $149.00
The second option would be to send your drive off to a data recovery specialist such as Ontrack to allow them to recover your data for you http://www.ontrack.com
Let us know if you need more info.
There are a few possible reasons your upgrade may have failed which is why it is reccomended you back up prior to any major software changes, It may be that the upgrade hung up and when you resumed it disarded your old files as fragmented or orphaned to be honest short of examining the upgrade logs if they exist I could not be sure.
To recover your files using the software (which I am confident should work for you) you will need an external hard drive such as a USB drive or something like that as when you recover files they need to be written to a seperate space from where the files are trying to be recovered.
If you dont have an external drive and you have a second mac I can give you instructions on how to link them in Target Disk Mode so you can use the other Mac to recover files from this one.
Let me know.