Jar file could not find the main class

I just clean and built main project in netbeans, however the jar file will not run. It gives me the error: could not find the main class: game.HangmanUI.
I've already set that as my main class, the program works and everything just the jar file does not run.

How do I get it to run?

Boy are there a lot of people having this problem.
You need to do several things to provide info about your problem.
Open a command prompt, change to the directory with the jar file and enter this:
java -jar your-jarfilenamehere.jar

Copy all of the screen and paste here.

If your class is in a package, Open the jar file with some tool like WinZip and see that the path to the class file is the same as the package: game.HangmanUI

Also open the manifest file that's in the jar file, copy it and paste its contents here.

I just fixed it, turns out i had to put the datafile in the same directory as the jar :S i didnt realize thanks anyways, it helped actually cuz i needed to see the error from the cmd screen.

Some instance quite similar thing happen if your application or the NetBeans project has more than on main class. In such cases you've define the what's the main executable for JAR package on project properties.