This effect can occur if sufficient stored energy remains within the data-logger allowing it's CPU to remain 'alive' and in a 'sleep state'.  The data-logger's CPU will enter a sleep state as energy available to it depletes.  After removing the data-loggers batteries, its possible for stored energy within the data-logger to power the processor in a sleep state for up to 10 minutes.  Consequently its possible that the CPU can remain in a sleep state even after new batteries have been installed!

To resolve this effect, the data-logger's CPU needs to be reset.  Do this simply by removing the batteries from the battery compartment and allow the data-logger to rest for 10 minutes.  Re-install the batteries.  The CPU will reset, power up and behave normally.