dashboard and website *poof* GONE

I’ve spent time searching the forums but cannot find the right fix. I’m on the thesis theme.

I added css code in the custom file editor to center my nav bar from a reputable site that had lots of (apparent) successes. But I think I must have royally messed it up because now my site is completely gone, and so is my dashboard. Nothing is showing up at all. It’s just a big white blank nothing. I had previously (and successfully) added a signature to the same css code.

Is there any hope? I have zero idea what to do after hours of searching for a cure.

Thanks
Keri Mae