I live in the same region as you and our pond has also lost a lot of water. Probably around 20mm over the same period as yours. I'm not an expert on ponds, but this happens quite a lot to ours during the year. We just top it up from the tap and the fish survive OK.

The pond is about twic the size of yours and has a lot of plants in it as well as about 35 fish (mainly goldfish).