Let me share what worked for me:
When we do "mvn clean" - we are cleaning old compiled classes please lgo to ocation "your project directroy -> target " you will find this folder is getting cleaned because that's where MVN places it's compiled artifacts
Since it's already a MAVEN project then go to your project folder and open in command prompt. And issue "mvn clean" and then "mvn test" -> "mvn test" command will place all the compiled files under proper folders and then you can run your tests through TestNG or MAVEN itself.
Make sure you set up "M2_HOME" or "MAVEN_HOME" to your env path , sometime "MAVEN_HOME" doesn't work so add "M2_HOME" as well (google it for this set up)
Above suggestions worked for me so I wanted to share, good luck