。
She spent the weekend alone.
Literal
She [topic-は] that weekend [object-を] one-person [in a state of-で] spent.
過ごす ('spend time') takes the time period as direct object — [time]を過ごす is the standard frame for 'spent the [period].' 一人で uses で in its means/manner/state sense ('in the state of being alone'), describing how the time was spent rather than where. 一人 carries layered cultural overtones in Japan — pairing with 自分の時間 ('time for oneself') for positive solitude, or 寂しい ('lonely') for the negative side. Context shapes which reading dominates; this bare sentence leaves it ambiguous.