I am new to Nginx so please bare with me - i am using Nginx on WHM but all of a sudden today my site was very busy and i got 500 Internal Server Error nginx.

Any advice on what to do please

This could be anything. Maybe your PHP(?) is running out of memory, so you'll have to configure "memory_limit" in php.ini to a higher value. Just one idea. Without any further information it's impossible to guess.