。
Helen's words suddenly gave me strength.
Literal
Helen [gen-の] words [by-で] I [topic-は] suddenly regained-strength.
で here is a causal/source marker — 'because of / by means of / owing to' Helen's words. 力づく is an intransitive verb meaning 'to regain strength / take heart,' composed of 力 'strength' + づく (the 付く attachment suffix voiced after a vowel-final element). Note how Japanese prefers to name the cause of an emotional/physical shift in a で phrase and keep the experiencer as the grammatical subject, rather than making the cause the subject (as English does: 'Helen's words gave me...').