Control panel not responding

Hi.  Probably should have left the vehicle connected to the mains, but been away for a few days and had a low battery warning.  By the time I came back the electronic step had stopped working.  Connected the power and let it run for almost 24 hours - but the control panel is not powering up - and i have a Truma error message of W255 H - which I think just means it isn't getting a message from the control panel.
Battery is now fully charged and no obvious fuse/trip that i can see.
Control panel is showing date, temp, humidity etc. but won't turn on. 
Any suggestions?

This error message can occur if you disconnect mains hook up without first turning off the control panel. Always turn off the panel first.

There is a simple fix but I’m stuffed if I can remember what it is. You could check out old threads via the search facility and don’t forget there is a Factsheet section.