500 internal server error nginx

Our website is having some major issues and we need help urgently. I appreciate any one's insight into this!

We use the headway theme with wordpress. Our site is very image heavy and while I don't remember using NGINX while we were building, it seems as if we are!

Now, when I try to log into the back end of wordpress it give me this error:

500 Internal Server Error
----
NGINX

And now the whole website is down!

So I am assuming it has something to do with NGINX? I have no idea how to fix this! We have rebotted our server, we have re-installed a back up, etc.

HELP! DESPERATE!