First, open that jar file with winzip/winrar and see if manifest file is there. Then check your main class (The class which has the "static main" method). You should write it with its package name. So if it is class X in Y package, you should write "Y.X" in your manifest file as the Main-Class.
Read the tutorial below for more information:
Setting an Application's Entry Point (The Java™ Tutorials > Deployment > Packaging Programs in JAR Files)