I was trying to run mvn commands for PetClinic Spring Application available at https://github.com/spring-projects/spring-petclinic
The problem was Firewall setting and hence my mvn command was not working. It failed to download. In this case where you from behind a proxy, no dependencies will be downloaded.
Go to apache-maven-3.3.9/conf
Right click on settings.xml file
Locate <proxies> block in the settings.xml
Uncomment the <Proxy> Block and give value based on your Proxy Settings. Below is just a Sample.