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 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.
3. There you will see the ‘Dedicated IIS Application pool’ option.
4. Click on it and Switch OFF and Switch it ON once again.
5. Now try to load your site.