Not sure if this is the right place for this but here goes.
I have a linux machine running Ubuntu 16.04LTS that's only occasionally used for distributed computing. Last time I shut it down it was working perfectly. That was about 6 months ago. I tried to restart it yesterday but it wouldn't boot. I ran a boot recovery disc and it will now boot but I have no internet connection. I have several other non linux devices running so I know my internet is good. I've tried several of the suggestions I found while searching the net but none of them worked.(I know,should have come here first) Test on the NIC shows it's functioning as it should. Tried a brand new ethernet cable and even plugged into a different router port. Nada.One suggestion was to upgrade to 18.04 or 20.04. I downloaded the 18.04 ISO (twice) and burned it to disc (twice)but when I boot to it all I get is a black screen. The files are being loaded but it doesn't help if I can't see what to do. Sure would like to get this going without having to wipe the drive and do a complete reinstall.

What do you mean by "no internet connection" - how did you test this?

Does this computer have access to the network? I.e. can it reach routers or other devices on the same network to which it is attached?

Assuming that your internet connection is wired (you do mention cables), I would check (this is not complete and not necessarily in the right order, but it's a start, I'd hope):

• Are the interfaces listed by commands like ip link?
• Do they have IP addresses (ip address)?
• How are their IP addresses configured? On Debian 16, this is normally done in the file /etc/network/interfaces, or files under /etc/network/interfaces.d.
• If they do have IP addresses, check the routing table (ip route). Normally, the default route tells the computer where to send traffic that goes to the internet.
• If IP addresses are configured, check basic connectivity to devices with ping. This includes addresses on the internet, e.g. ping 1.1.1.1.
• If by "no internet", you actually mean that the computer can't resolve domain names, e.g. ping google.com responds with an error about not knowing google.com, then you may well have internet access, but your domain name resolution doesn't work.

This linux machine can be seen by my main desktop which I use to control it when running distributed computing. The Linux machine has an assigned static IP address as do all my computers. When I ping the Linux box from my main windows box I get normal results. When I ping the other direction I get nothing. When I ping google.com from the Linux box I get unknown host. It's like packets can go in but not out. I noticed something in the windows app log of the program I use to control all the other machines.World Community Grid 3/3/2021 9:04:59 PM Scheduler request failed: Couldn't resolve host name I've seen that a time or 2 over the years but in the past a reboot of the linux box cleared that up. Not this time. I checked the hosts file and the hostname file and they are the same. You could be right about the domain name resolution. How do I fix that?

I am not sure, but it could be due to an ill-configured routing table on the Linux box, such as the absence of a default route.
You have to configure DNS on the Linux box. This is done by putting the IP addresses of nameservers into /etc/resolv.conf.

But first, ensure that the Linux box has connectivity with local devices.

I'm 99.9% sure the static IP address is configured correctly including the preferred DNS servers. I've checked them against my other machines and except for the last # in the address they are the same.
The nameserver in the /etc/resolv.conf file is 127.0.0.53. From what I've researched that is correct. Just for giggles I added the preferred DNS server IPs to the file and I now have internet connectivity. Go figure. I don't remember doing that when I first set this machine up but who knows. My memory ain't what it used to be. I rebooted just to make sure and it's good. Should I just leave the 127.0.0.53 in the config file or does it matter if it's in there or not? Thanks for the help. You guys rock. I'll edit the title to solved.
Just another quick question.Is there any explanation as to why I got a black screen when I tried to update and is there a work around in case I want to upgrade later? Thanks again.

You need to learn how systemd-resolved is configured. You can start with the official man page, although plenty of explanations can be found in the internet.