Monitor without Batteries Won't Power Up When Rowing/Skiing
Your PM5 does not have batteries installed and it won't turn on when you are rowing/skiing (the act of rowing/skiing should power the monitor).
This is a known issue on PM5s with hardware versions 500–599 and Firmware Version 20/720. To resolve this issue, update your PM to Firmware Version 22/722 or higher.
To determine which hardware and firmware versions your PM5 is running, from the PM5 Main Menu, select More Options > Utilities > More Utilities > Product ID.