Change the cmd/command window prompt to the directory that contains the classfile to be run, and issue a command like this:
java -cp . <nameOfTheClass>
Include the period and the surrounding spaces. Do not include the .class suffix in the command.
If that works, it was a classpath problem. If you want to use other forms of the command, read and learn about how to use the classpath, and how it is set. See the java tools (java, javac, etc) documentation for that.