Since DNS is not hosted on your droplet an upgrade or even downtime on your droplet would not cause the error you are seeing. Can you share the domain name you are encountering problems with? There are a few possible reasons you may see this error:

• Your domain has expired so your registrar is no longer routing requests for it to DO's nameservers
• There is a misconfiguration in your DNS record and it is returning an incorrect IP address
• Your ISP's DNS resolvers are experiencing issues causing them to fail to resolve the domain
• The changes you made to your DNS settings have not yet propagated to your ISPs resolvers.
  

The second or last item here are the most likely based on your recent changes. To troubleshoot the second option, check your DNS record to ensure the IP is correct. For the last item, the best resolution is found by waiting for propogation but you can test this by changing your local DNS resolver to a publicly provided one like Google's 8.8.8.8 and see if the domain then resolves. If it does, you will most likely just need to wait for your ISPs servers to pull the updated record.

Thank you for the reply.

Here is the link: https://www.phildiscuss.com

1. My domain will expired on Nov. 2018.

2. DO, domain config are:
   A-@-159.65.132.141
   AAAA-@-2400:6180:0:d1::1ae:9001
   ns1.digitalocean.com
   ns2.digitalocean.com
   ns3.digitalocean.com

And on my domain registrar, I only put the nameservers (ns1.digitalocean.com,ns2...,ns3...) as record there.

1. IDK

2. It's been 6 days, I hope 6 days is enough to propagate.

Thank you.

Hello. Thanks for the reply.

URL: https://www.phildiscuss.com

On my domain registrar the only record that I put there is the Nameservers(ns1.digitalocean.com, ns2... ns3...)

On my DO, DNS Record are:
AAAA

phildiscuss.com
2400:6180:0:d1::1ae:9001
A

phildiscuss.com
159.65.132.141
NS
ns1.digitalocean.com
ns2.digitalocean.com
ns3.digitalocean.com

It's been 6 days, I hope 6 days is enough to propagate. LOL!

Thank you