The posted reason from the CME Group was "cooling issue". However, one of the lightest trading days of the year does not cause much in the way of heating. Plus, there is triple redundancy in cooling, and they all "failed". All futures trading was halted, including oil and stocks, in addition to PMs. Then, once it was being restored, all the non-PM futures came back first, while silver lagged.
it was in the middle of the night ...