"If not, what then?"
Übersetzung:Wenn nicht, was dann?
Was ist die Unterschied zwischen "Wenn nicht, was
dann?" und "Wenn nicht, was
denn?" in dieser Kontext?
"Was ist der Unterschied [nominative] zwischen "Wenn nicht, was dann?" und "Wenn nicht, was denn?" in diesem Kontext [dative]?
'dann' is a time word. Here it is also used to ask for an alternative after the first solution failed. The alternative can be an object like a cup or a car, it can also be an activity...
'denn' is a word which is used to say 'in contrast to something'.
I think it is a personally decidion what one preferes. (In general) 'denn' has many meanings. I tried to kept it easy.