Want to restore iphone from old backup?

I lost my iphone and then got it back, but someone erased my phone. No big deal, I backed it up. So I started to back up on itunes but I had to go to work. I got back and put it into itunes again, but without me realizing it, it made a new backup with this empty phone. How do I find my old backup? It's not in the list of backups and I'm freaking out, does a new backup just override my last backup and make it unavailable?

If your phone wasn't connected to Wi-Fi or 3G, you could try iCloud backup, it has an option on phone setup, otherwise, I think you are royally screwed.