The sentence describes two actions that happened in the past.
The first action, The accident took place, is a short, completed action in the simple past tense.
The second action, connected by when, was an ongoing action that was interrupted by the accident.
To describe a past action that was in progress when another action occurred, we use the Past Continuous Tense (was/were + verb-ing).
The subject is she, so the correct verb form is was crossing.
The full sentence is: The accident took place, when she was crossing the road.