Need help in current situation

My TA is currently out of office due to medical reasons and identified me as the backup without a proper handover. As a result, I wasn’t included in several initial communications and wasn’t aware of the Sprint 0 activities, which prevented me from starting it on time.

Now, the client is expressing concerns about the delay and is threatening to escalate the matter to the DP and BRM. I’d like to understand the best way to handle this situation, given that the transition and context were not properly shared with me.