Ubuntu: DNS server not responding, unable to resolve host - temporary failure in name resolution

1. Check 2 files /etc/hostname and  /etc/hosts 

sudo nano /etc/hostname

samuel

sudo nano /etc/hosts 

127.0.0.1       localhost localhost.localdomain samuel
127.0.1.1       samuel-ubuntu
127.0.1.1       samuel

2. Make sure the name is the same in both files

3. Check content in /etc/resolv.conf file

nano /etc/resolv.conf

4. Add nameserver to the resolv.conf

nameserver 8.8.8.8

5. Restart service

sudo systemctl restart systemd-resolved.service