Scheduling Issues Monday Hour 1 – Not sure how to proceed.

I have done the Monday Hour 1 course a few months back. I am a Web Designer and Online Business strategist for Health & Wellness/Life Coaches. There are a few times when client websites are having technical difficulties and I cannot either wait or schedule those events. When this happens, there is no way to know in advance if the issue will take 5 min or 5 hours to fix. When this happens, then my schedule cannot be honored. Do I simply reschedule the rest of the day/week? How can I factor those events in? Any suggestions?
Thank you!