Bluetit abandoned nest?

Hello. I have a nest box with camera. We had a blue tit lay 9 eggs. They all hatched but within a week, 2 had died. The others all seemed strong and healthy. Since yesterday morning, another 3 have died. I can see the bodies of 2 at the edge of the box.

There are 4 left now that are 12 days old. They have wing feathers and fluff and have just opened their eyes. 

I check them all the time. Mum is a single parent. She has been feeding them all day as usual but then suddenly stopped returning to the nest. It's been 2 hours now and it's starting to get dark.

They still seem strong but are calling out for food. I don't know if mum has abandoned them or if she can't get back to them. I'm so worried.

Should I ring our local wildlife hospital? They must be hungry and probably cold without her now.

Am I overacting?