My default JRE got changed when I was configuring my Eclipse to work with Google App Engine. This resulted in an error in Netbeans as it was not able to find the old JRE. To fix this you need to do this,
Go to [Netbeans installation directory]/etc/ folder and open the netbeans.conf file.
in that find this line and,
netbeans_jdkhome=”set the path to the new JRE folder here”
Save and restart/start netbeans. 🙂