Well, being buried in snow is better than that temperature without snow. If you had a Christmas tree or your neighbors put one out, cut branches and put them over your Erica until the temperature is warm enough to not have freeze/thaw cycles. I'd wait until spring and bloom time is done, and then if you have clearly dead parts, cut them back. Also you can trim them after bloom to shape them. I wouldn't trim them now.

NHBabs advice is good. I will only stress the importance of her suggestion to cover the plants with evergreen boughs, but do so after the ground refreezes. During this thaw, check to see whether or not the plants have heaved their roots out of the ground. If they have, tamp them back into place.

Now the bad news. Erica darleyensis is not hardy in zone 5. However, Erica carnea, also flowering in late winterÂearly spring, is. And with winter protection, so might be Erica cinerea, which is blooms in summerÂfall. Reliably hardy are most cultivars of heather (Calluna vulgaris), which bloom from late summer into fall. All require the same horticultural conditions.

Buy from reliable sources, and never trust the tags from mass-market nurseries or plant sellers like Home Depot or Lowes. A reliable source east of the Rockies is given below.

Here is a link that might be useful: Heaths and Heathers