org.zefer.pd4ml.PD4ML is the main class of PD4ML. The message tells, that the class is missing or not in the classpath. I have no idea what could happen on your side, but please make sure pd4ml.jar, ss_css2.jar and optionally pd4ml_tl.jar still exist on the server and listed in the classpath or located in WEB-INF/lib directory.