You're right Mary Stella, if temps are down to 10*F or below and there is no snow cover I've seen damage to blooms. But that has been daffodils and hyacinths I saw that happen to, the crocuses and snowdrops seem to weather those temps well.
I've also seen damage when cold temps (low 20s) follow a period of unseasonable warmth. Friday was very warm here, 77*, which brought on rapid growth on the bulb plants, and was followed by a low of 23* Saturday night. The tender growth of stems of some of the daffodils looked to be damaged by the cold. Interestingly, the blooms look fine.