Fermentation stopped prematurely

[gilead999]

Hi all,
I am brewing a Nut Brown Ale, I have my fermenter in a big plastic box. I put a few frozen water bottles around it, I changed the water bottles for more frozen ones yesterday but the airlock stopped bubbling away, since the temperature dropped to around 18 degrees. Does anyone know if this has "shocked" or perhaps killed the yeast? Or will it reactivate now that the temperature has risen naturally to about 21 degrees?

 

[Yob]

S04?

 

[ArgM]

What do the gravity readings say? if they are still changing then the little fellas are just working a bit slower!