Learn German: Auf einmal war es ganz dunkel. - All of a sudden it was completely dark.

Auf einmal is a common fixed phrase that translates to "all of a sudden" or "suddenly". It means the exact same thing as plötzlich and they can be used interchangeably. Auf einmal is slightly more conversational but perfectly fine in everyday spoken and written German.

Why is the verb before the subject (war es instead of es war)?

In German, the conjugated verb must always be in the second position in a standard main clause. Because the time phrase Auf einmal takes up the first position, the verb war must come right after it, pushing the subject es into the third position.

I learned that ganz means "whole", like in die ganze Welt. Why is it translated as "completely" here?

When ganz is used as an adjective before a noun, it means "whole" or "entire". But when it is used to describe another adjective (like dunkel here), it acts as an adverb meaning "completely", "totally", or "quite". So ganz dunkel means "completely dark".

Why does this sentence use the simple past tense (war) instead of the spoken past (ist gewesen)?