。
She got angry when she realized the door was locked.
Literal
She [topic-は] door [on-に] key [subject-が] is-applied [nominalizer-の] [subject-が] understood [because-て], [got-angry-腹を立てた].
Several pieces stack here. ドアに鍵がかかっている is the standard idiom for 'the door is locked' — literally 'a lock is applied to the door,' using the passive-leaning intransitive かかる ('to be hung/applied'). The whole locked-door clause is then nominalized with の and made the subject of わかる ('to understand, realize'), which takes its understood thing with が. The te-form わかって gives a cause/reason link to the main clause 腹を立てた ('got angry'). 腹を立てる is a body-part idiom — literally 'to stand the belly up' — for getting angry, part of a productive family that uses body parts to encode emotions.