HTTP ERROR 500 - Can't access my site

Greetings Everyone - My site has been having some heavy traffic recently, but now I am getting a message "currently unable to handle this request."

First thing that comes to mind is to clear cache but I can't access my una/studio either.. Please note, I have full root access to my server where my site resides, but I am not sure what folders to clear & how to fix this issue.. I also have full access to WHM, I was wondering if there is anything I can do from WHM too..

Any input is much appreciated.. Thanks.