It seemed some troper thought that this episode's Arendelle scenes happened right after the last episode's (right after Ingrid left for our world. This is not the case. Arendelle was frozen for over 30 years. There're many evidences supporting this:
Hans mentioned Arendelle was frozen for over 30 years.
Double as a Call-Back, Blackbeard mentioned being forced to walk the plank. Hook did it in "the Jolly Roger", a season 3 flashback after the end of the first curse and beginning of the second curse.
Lastly, the scene where Elsa wished for Anna to returned pretty much confirmed that they happened at the same time. An amazing stroke of luck for the two sisters, had it been a little longer, they may never be reunited.
In conclusion, it would seem Ingrid unfreezed Arendelle the night she casted the Spell of Shatered Sights. The reason is either she needed all her magic to cast it or now that her plan is complete's there's no need to freeze Arendelle any more.
The time Arendelle was frozen.
It seemed some troper thought that this episode's Arendelle scenes happened right after the last episode's (right after Ingrid left for our world. This is not the case. Arendelle was frozen for over 30 years. There're many evidences supporting this: