• Accounting resources at month-end
• Expedited contract review during an acquisition
• Stakeholder change of mind moving a deadline up 3 months
• Key developer, main business sponsor, funding pulled after 3 months of requirements refining

I have experienced all of the about while managing projects.  I allow about 5 minutes to let the shock wear off and then:

• Find out solid dates and/or availability; mentally shave off a few days as no one stays until 5pm on Friday if they are quitting their job
• Assess impact of solid information and also for gut feeling or hearsay
• Discuss changes with management
• Inform stakeholders of impact with updated plans and potential scope adjustments in hand
• Other necessary and precautionary measures

You cannot control decisions of others but you can bring the results and mitigate for the best outcome.