title: jenkins critical error after plugin update
body: <p>I attempted to update a couple of my jenkins plugins (don't remember which) but got hung. After about 15 mins, I decided to restart the jenkins server. This is what I get on the jenkins UI:</p>
  
  <pre><code>org.jvnet.hudson.reactor.ReactorException: java.lang.Error: java.lang.reflect.InvocationTargetException
      at org.jvnet.hudson.reactor.Reactor.execute(Reactor.java:246)
      at jenkins.InitReactorRunner.run(InitReactorRunner.java:43)
      at jenkins.model.Jenkins.executeReactor(Jenkins.java:894)
      at jenkins.model.Jenkins.&lt;init&gt;(Jenkins.java:796)
      at hudson.model.Hudson.&lt;init&gt;(Hudson.java:81)
      at hudson.model.Hudson.&lt;init&gt;(Hudson.java:77)
      at hudson.WebAppMain$2.run(WebAppMain.java:214)
  Caused by: java.lang.Error: java.lang.reflect.InvocationTargetException
      at hudson.init.InitializerFinder.invoke(InitializerFinder.java:124)
      at hudson.init.InitializerFinder$TaskImpl.run(InitializerFinder.java:184)
      at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:259)
      at jenkins.model.Jenkins$7.runTask(Jenkins.java:883)
      at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:187)
      at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:94)
      at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
      at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
      at java.lang.Thread.run(Thread.java:662)
  Caused by: java.lang.reflect.InvocationTargetException
      at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
      at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
      at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
      at java.lang.reflect.Method.invoke(Method.java:597)
      at hudson.init.InitializerFinder.invoke(InitializerFinder.java:120)
      ... 8 more
  Caused by: java.lang.NoClassDefFoundError: hudson/PluginManager$PluginUpdateMonitor
      at hudson.maven.PluginImpl.init(PluginImpl.java:54)
      ... 13 more
  Caused by: java.lang.ClassNotFoundException: hudson.PluginManager$PluginUpdateMonitor
      at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
      at java.security.AccessController.doPrivileged(Native Method)
      at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
      at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
      at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
      ... 14 more
  </code></pre>
  
  <p>I attempted to update the war from 1.486 to 1.502 but no change in behavior. What can I do to get Jenkins operational again?</p>