G'day all

I'm in the middle of a weekend workshop and one student's N5100 has started to misbehave

It permamently displays "Subject is too dark" > full info screen is shown below


I /we have gone thru the menus & tried to reset to factory config > can't find a menu option so worded but two are similar
I /we have swapped lens with 2nd body > same result, same msg

So - my Q is .... how can I fix / reset this or is it a sign of something worse???

I would like an answer before things start again at 0900 sunday morning 17th
any answers will help - we'll keep logging in thru the day, but the day is busy enough with scheduled materials

Regards, Phil