This is what one would consider a known-unknown from a risk management perspective. Used to be called 'act-of-god'. Your risk management plan register should have a line item for this and your project plans (cost, time, quality, resource, etc) should have addressed it in general terms.
More specifically you have to look at the individual impacts on each project element (micro-impacts) as well as the overall effects (macro-impacts). Yes, there will be negative impacts but your job as a project manager is to mitigate (make the best of it) and advise up the line.
However, the risk management plan should include a response plan for unexpected events. You may not have a specific response but you need a strategy or process to respond to any eventuality.
1) convene the risk team and barnstorm over possible impacts
2) report up the line - client, stakeholders - providing a preliminary general plan to respond - if anything gives them comfort that the threat has been identified and the team is developing a specific response
3) Look at the impact on each project management element, assign specific staff to analyze the extent of the impact, and develop specific mitigation measures.
