"Where were you?"
Translation:Wo warst du?
woher means from where.
you could ask woher bist du (present) = from where are you (present simple)
but it would not work in English when using the past simple: from where were you?
The German präteritum, being the equivalent to the English past simple, would thus work neither with woher.
an exeption might however be:
woher warst du nochmal? = from where where you again - where did you say you were from?