Tiny Towers is rated as hardy down to the 0 to 10 degree range. I don't think the cold is the problem. If they were actively growing during warm weather and then a very sudden drop to 15 occurred I guess some damage is possible but this is unlikely.
Browning could be due to something sprayed on them or that drifted on them. In summer mites are a possible cause and certain needle diseases can also cause browing especially when the plant is wet frequently by sprinkler irrigation. Otherwise there is not another common cause that I can think of. When junipers and cypress plants turn brown and lose all needles in a section that section usually does not regrow. I would give them some time and wait for warming weather to see if they can pull out of it and regrow before giving up on them.
One final thought is soil moisture. Both dry soil and soggy conditions can damage roots and lead to problems such as you described. The damage can be long term if the problem is not corrected quickly. In container such soil moisture extremes are more common.
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