How to get my user data back?
The easiest way to find out where all this data is, would be to go to the root of the partition in Finder, then hit Cmd+Alt+i. That will bring up the Inspector that should tell you that, yes, your partition has 80GB of data on it.

That's not particularly helpful in itself, but if you then select the folders contained within "/" that are visible ("Applications", "Library", "System" and "Users") you can see the corresponding sizes of those folders.

If "/Users" is huge, then it's possible that your data is still there but hidden for some reason. If it is, I would suggest rebooting to your Recovery Partition and resetting your User folder permissions and ACLs.

As a good measure, I would also repair your Disk Permissions from Disk Utility in the Recovery Partition too, just in case there's extra fluff getting in the way.