"The Lost Woman" drops the reader right into the traumatic event that inspired Beer to write the poem: the sudden loss of her mother at a young age. (Beer's own mother died of cancer when the poet was 14.)
The speaker begins by saying her mother "went with no more warning." The word "went" is deliberately vague; the poem doesn't immediately reveal the mother's fate, creating suspense that mirrors the speaker's own shock upon coming home to find her mother gone. The enjambment of line 1 creates yet more anticipation: where, exactly, did the mother go without warning? Readers have to jump to the next line to find the answer to this question.
What's more, "went" implies that the mother had some agency here—that she chose to leave her daughter behind. This isn't the case, readers will learn in the next line, but already the poem hints at some of the bubbling resentment and discomfort lingering beneath the surface of this mother-daughter relationship.
In line 2, the speaker clarifies the situation—somewhat: the only "warning" her mother gave about her imminent departure was "a bright voice and a bad pain." By "bright voice," the speaker might mean that her mother remained cheerful or level-headed to the end. (That "bright voice" might also evoke a sharp, sudden gasp of surprise.) A "bad pain," meanwhile, hints at the mother's illness.
The speaker was "Home from school" (either coming home after class or out for summer break) when she saw the "back of a shocking white / Ambulance" driving away from the gate to her home. That "shocking white" color reflects the speaker's own "shock" at the situation. She did not expect to come home to find her mother being carted away in an ambulance.
This stanza establishes the poem's form: the poem is written in sestets (six-line stanzas) with an ABABCC rhyme scheme. This pattern can be hard to hear at times, however, because many of the poem's rhymes are very subtly slant rhymes ("white" and "gate," "began" and "down," etc.). Its music is quite subtle.