How to solve website not loading or 503 error on website

This usually happens due to application pool is getting crashed, it mostly crashes when if there are a huge number of real-time visitors or may be a bug in a code that might be creating a loop, please verify your code once.

You can also restart your application pool for a particular domain using the following steps

1.  Login to Plesk control panel.
2.  select a site that you want to restart the application pool.