WS-250-DC went to sleep wont wake up

[wisp-wand]

We have multiple WS-250-DC in our network and one that has been in service for over 5 years without issue until now. 24VDC battery system. It has the power down / hibernate setting at 22v.
There was a power cut at the site and one of the 12v batteries in series had failed. This caused a rapid drop in voltage to around 14vdc and the Netonix correctly shutdown / hibernated.
Wake up voltage set to 25vdc.
When power came back on, voltage up 28v netonix won't wake up. The power board is on as the surface mount LED's are lit, but no power to main board.
I replaced with a new unit and recovered the site. Back at the office, I've tried 28v, 48v and leaving off for a few days, but no joy.

Any suggestions as to what I can do to wake up this unit, or is it done?

Thanks

W