We had -30C a couple nights last week and I noted one of my 2 hives was not making frost at top entrance. 
Today we had +10C so I went in. Total loss, all bees still in the hive, all dead, looks like starvation. Hundreds, (thousands?) heads buried in cells.
2 outer frames in bottom deep full of honey, 5 frames of honey in upper deep still full, sugar board above that almost untouched. 
No mite frass, no brood at all, no evidence of DWV - looked at hundreds. Bottom board was hellacious - thousands of dead bees, clustered towards back centre of hive. 
I am certain it was starvation, but why? Could it have been too cold for bees to access the outer stores? 
We have had highly variable temperatures this winter. 
Hive next over was doing cleansing flights and various bees committing sideways in the snow. Huge activity, gives me hope, and some trepidation. 
So, no obvious reason for the starvation, but definitely starvation! Hive went into Winter with a 10 deep super of honey on top and 4 frames honey in bottom deep.
Sugar board installed above in November, before solar wrap and insulating cover went on. 1 for 2 so far. Wish me luck.
About 53N 115 W 900M. West Central Alberta

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