Looking through the list of Satutory Instruments coming into force , I discovered this rather strange item (pdf) :

The order was rescinded (by another SI) at 13:15 on the 20th December with the following :

That emergency having now passed, the Secretary of State no longer deems it necessary in the
public interest to restrict flying in that area and makes the following Regulations in exercise of the
powers conferred by Article 161 of the Air Navigation Order 2009(a).

Interestingly, I can find no mention of a forced closure in the press.

The only item from the 19th mentioning London City Airport is as follows from The Telegraph (supposedly valid at 17:00 on 19th December) :

London City Airport: London City Airport is open but there are disruptions and cancellations. Passengers are advised to check the weather conditions at their destination airport. Click here for airline contact details.

My first though was that the order was enacted to prevent all the aircraft who couldnt get into Heathrow or Gatwick making a beeline to London City but as it only has a short runway, very few types of aircraft can actually land there which leaves me none the wiser as to what the emergency actually was.