How Would a Sudden, Significant Drop in Network Hash Rate Affect the Security of the Blockchain?

A sudden drop in hash rate would temporarily make the network significantly easier to attack. Since the difficulty adjustment is delayed (e.g. two weeks for Bitcoin), the cost of a 51% attack drops drastically during this period.

An attacker could acquire the necessary hash power at a much lower cost, making the network highly vulnerable until the next difficulty adjustment restores the intended security level.