JSF problem - java lang NoClassDefFoundError

[dave sag]

Hi, I am by no means a newbie now with JSF but have hit this strange and very newbie type issue. Our web app was working fine and then, with seemingly no changes, we are getting the above NoClassDefFoundError looking for javax/faces/context/ExternalContext

java.lang.NoClassDefFoundError: javax/faces/context/ExternalContext
at com.sun.faces.el.PropertyResolverImpl.getValue(PropertyResolverImpl.java:75)
at com.sun.faces.el.impl.ArraySuffix.evaluate(ArraySuffix.java:162)
at com.sun.faces.el.impl.ComplexValue.evaluate(ComplexValue.java:146)
at com.sun.faces.el.impl.ExpressionEvaluatorImpl.evaluate(ExpressionEvaluatorImpl.java:238)
at com.sun.faces.el.ValueBindingImpl.getValue(ValueBindingImpl.java:155)
at com.sun.faces.el.ValueBindingImpl.getValue(ValueBindingImpl.java:136)
at javax.faces.component.UIComponentBase.isRendered(UIComponentBase.java:308)
at javax.faces.webapp.UIComponentTag.isSuppressed(UIComponentTag.java:828)
at javax.faces.webapp.UIComponentTag.doStartTag(UIComponentTag.java:460)

now clearly tomcat (5.0.19) is loading the javax.faces stuff as further down the stack trace there are javax.faces.webapp.UIComponent classes being loaded okay. we have ensured there are no jar file conflicts, etc etc but just can't get past this issue. anyone else had this issue and able to suggest a solution?

dave