This usually happens due to application pool is getting crashed,
it mostly crashes when if there are the 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 following steps
- Login to Plesk control panel.
- select a site that you want to restart the application pool.
- There you will see ‘Dedicated IIS Application pool’ option.
- Click on it and Switch OFF and Switch it ON once again.
- Now try to load your site.