How do you handle BAS commissioning when the controls contractor has already left the building?

Walked into a job last week — building was 2 years old, BAS had never been commissioned correctly, controls contractor had been out of business for 14 months.

Owner wants me to “”fix it.”” Where do you even start when there are no as-builts, no trend logs, no commissioning report, and the original sequences are guesses?

Looking for the war-story-guys playbook.