Produce a disaster management plan while the waters are calm, so that you are prepared should the worst happen. As part of this plan, you should have clear delegated authority, particularly in relation to decision-making by the charity's chair and senior staff. Make sure that a clear record is kept of how this has been followed.
Inform the rest of the trustees as soon as possible, using suitable means of communication - whether that's by telephone conference or email, or by calling an emergency board meeting. When the board does meet in person, it might wish to ratify decisions made, if appropriate, and decide any follow-up action required.
Produce a communications plan covering both external and internal audiences. Preparing reactive and proactive statements can help, but make sure that key stakeholders - including trustees - are clear about who is giving what message to whom, and when.
Once you're out of the woods, reflect on the situation and ensure that the decision-making has been properly documented. Update policies and procedures with lessons learned.