How to restore hostname for server?

Today,

I made some small settings with /etc/hosts & /etc/sysconfig/network for sending emails

I really I didn't make some changes, but now all sites on server can not access, but sentora panel is okay

May you help me with this? How to restore hostname to resolve the sites?

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seems now I can not ping to all sites in server, only ping to sentora panel

mydomain.com’s server DNS address could not be found

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https://intodns.com/prescriptz.com shows too many errors about DNS & NS although I didnt change, may you help me?