Broken Key

"Why did you indicate that waiting until the morning would be better?" asked Mr. Schloss.

"Some authorities question what we mentioned," replied Rabbi Dayan. "Furthermore, if the case were about clipping a padlock, the issue would be simpler. However, since we are dealing with breaking a permanent lock, there is concern that the owner might have some objection. Nonetheless, the Gemara [B.K. 114a] indicates that in certain situations a person is allowed to cause minimal damage to another, with reimbursement, to spare himself a great loss. Thus, it would be preferable to wait and get the key or the landlord's explicit permission, but in cases of need it is permissible to call a locksmith." (Rama C.M. 274:1)

(See Emek Mishpat, Sechirus Batim 36:27; Mishpetai HaTorah 1:14)