domain name system - Multiple sites on server but only one is down. Possible DNS issue?

I have several sites on a VPS server and one of the sites randomly went down earlier (500 error). The site is hosted on a subdomain of another website that is hosted elsewhere. So, the site that is hosted on my server is blog.example.com while example.com is hosted on another server.

I think the issue is DNS related and possibly NS record related. Prior to the site going down, I had the subdomain website NS records set as my server NS records e.g. ns1.myserver.com and ns2.myserver.com. I thought I might need to change the NS records to the server that is hosting the main website example.com. I changed the NS records to ns1.example.com and ns2.example.com but the problem is still not fixed. Could there be a different problem or another solution?