How to Manage Customer when last minute Critical issue faced just before UAT

Good to have detail discussion about last hour crtical issue with client,It is obvious in software develoment probablity of risk will trigger is unexpeced ,Therefore as manager we will have a contigency plan to mitigate the unknow unknow risk.As manager i have to use the contigency plan(schedule and mitigation to overcome the issue) and inform client that using this plan there are chance of delivery as expected if not possible will allow other components to delivery ,keeping this aside.