On the rare occasion that this happens, you have two options. The first option is to reinstall the cPanel Varnish Plugin using the release package so as to restore Varnish to a pristine condition. The second option is to remove Varnish completely.
Option 1: Reinstall Varnish cPanel Plugin
Option 2: Remove Varnish cPanel Plugin
Keep in mind that while we make Varnish tuning available via the cPanel plugin, there is always a risk that your system might not cope well with the values you supply it. So be aware.