Lost all files after installing Yosemite I upgraded to OS X Yosemite and I have lost all my files or I don't know where they are or how to get them.

I didn't realise that upgrading would delete all my files. Is there a way to go back to retrieve these files? All my business files, pics and personal files are gone.
The Installation of new operating system will normally not touch your files.

What it might have happened is you created a new user account and all your files are in the old user account.

If you have admin privileges you should be able to see the old user account in sys pref users and switch to it.