No tftpd access after changing ubuntu IP

So DHCP is working?
But all computers get no TFTP boot IP?
Or rather they get an IP but cannot connect. If so, this might fix it:

If you changed the IP of your DHCP and TFTP boot server, did you also configure DHCP Option 67, 'next-server' TFTP boot server IP to the new IP of the DHCP server? This option is static and needs to be configured manually when changing the TFTPboot Server's IP in /etc/dhcpd.conf, for instance if your subnet is 192.168.0.0/24 and the tftp server 192.168.0.1:

subnet 192.168.0.0 netmask 255.255.255.0 {
...
# PXE Parameters
next-server 192.168.0.1;
filename "pxelinux.0";
...
}

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Yes I did configure this. Even though I restarted the service it didn't work until I rebooted the machine for some reason.

So is it working after the reboot?
If not, can you check TFTP is working with a tftp client and 69/UDP is not firewall blocked?

You don't have to specify "next-server" if the TFTP server is running on the BOOTP/DHCP server. In that case, PXE clients (rom) assume that the "next-server" is the dhcp/bootp server.
You may find some useful details in my article
https://www.experts-exchange.com/Networking/Misc/A_2978-PXEClient-what-is-it-for-Can-I-use-PXE-without-it.html
(and the comments I wrote to it afterwards)

You don't have to specify "next-server" if the TFTP server is running on the BOOTP/DHCP server.

True! But since you always need to define the filename parameter, I like configs to be clear. But the dhcpd.conf does not seem to be the problem here anyway.

In that particular case, not specifying the next-server would have been a "good" thing after the IP address changed.
However, I think the problem was solved by rebooting the server (at least this is what I seem to understand from what the author wrote lately)

Yes everything is working. Can anyone tell me the command to actually reboot. I couldn't find this and had to pull the plug!

Thanks