I am facing exactly the same problem with the latest versions of Gerrit and Jenkins - it is a bit confusing as I need to cancel or pause big number of builds with every Gerrit restart.

Jenkins ver. 1.518
Gerrit Code Review (2.7-rc1)
Gerrit Trigger plugin: 2.10.0

I have the same issue on Jenkins ver. 1.509.3 Gerrit Code Review (2.6.1) Gerrit Trigger Plugin 2.10.1.
It's confusing to stop all the jobs of my company to restart the server.
I can see NPE when we click on Stop or Restart (unfortunatelly I can't manage to find out the log).

I also saw this issue. Gerrit went down in the middle of the night and Gerrit Trigger was not able to reconnect, although I was able to restart Gerrit Trigger through the console without restarting Jenkins. I saw the errors below many times in the logs. It looks like the server was down for several hours and the Gerrit Trigger connection timed out eventually.

SEVERE: IOException:
com.sonyericsson.hudson.plugins.gerrit.gerritevents.ssh.SshException: com.jcraft.jsch.JSchException: java.net.ConnectException: Connection timed out
at com.sonyericsson.hudson.plugins.gerrit.gerritevents.ssh.SshConnectionImpl.<init>(SshConnectionImpl.java:126)
at com.sonyericsson.hudson.plugins.gerrit.gerritevents.ssh.SshConnectionFactory.getConnection(SshConnectionFactory.java:76)
at com.sonyericsson.hudson.plugins.gerrit.gerritevents.GerritConnection.connect(GerritConnection.java:365)
at com.sonyericsson.hudson.plugins.gerrit.gerritevents.GerritConnection.run(GerritConnection.java:293)
Caused by: com.jcraft.jsch.JSchException: java.net.ConnectException: Connection timed out
at com.jcraft.jsch.Util.createSocket(Util.java:341)
at com.jcraft.jsch.Session.connect(Session.java:192)
at com.jcraft.jsch.Session.connect(Session.java:160)
at com.sonyericsson.hudson.plugins.gerrit.gerritevents.ssh.SshConnectionImpl.<init>(SshConnectionImpl.java:122)
... 3 more
Caused by: java.net.ConnectException: Connection timed out
at java.net.PlainSocketImpl.socketConnect(Native Method)
at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:327)
at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:193)
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:180)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:385)
at java.net.Socket.connect(Socket.java:546)
at java.net.Socket.connect(Socket.java:495)
at java.net.Socket.<init>(Socket.java:392)
at java.net.Socket.<init>(Socket.java:206)
at com.jcraft.jsch.Util.createSocket(Util.java:335)
... 6 more
Feb 6, 2014 2:17:32 AM com.sonyericsson.hudson.plugins.gerrit.gerritevents.GerritConnection connect
SEVERE: Could not connect to Gerrit server! Host: myserver.com Port: #####

Just experienced this on Jenkins 1.583, Gerrit Trigger 2.3.1 (yes, old version, but from above comments, seems to exist in recent versions as well).

We could not reproduce this issue with Jenkins 1.577, Gerrit Trigger 2.11.1, Git Client 1.10.1 and Git Plugin 2.2.5.
The Gerrit Trigger plugin automatically reconnected to the Gerrit Server. Any further pushes to Gerrit resulted in properly triggered Gerrit voter jobs.

We also tested Gerrit shutdown while a Gerrit voter job was in the build step. Of course the Gerrit Trigger plugin could not vote when the job finished. This voting is lost.
And the Gerrit Trigger plugin did not trigger the voter job for this patch set again, but it has been triggered for all new patch sets. And for the lost patch set a manual retrigger worked as well.

@alexanderlink: how long was your Gerrit server down? In our case, it was down for several hours, and finally the Gerrit trigger just gave up, or became hung/wedged.

Confirming that for us as well it was down a few hours.

I'd also like to add that this isn't 100% reproducible even for us. Sometimes the server is down and comes back up with no issues, other times we encounter the issue in this ticket - it appears to be intermittent.