。
She's looking for the wallet she lost.
Literal
She [topic-は] became-lost wallet [object-を] is-searching.
なくなった財布 ('the wallet that became lost') is a relative clause modifying 財布 — built from なくなる ('to disappear, be lost,' the intransitive partner of なくす 'to lose'). The choice of なくなる over なくした is subtle: なくなった財布 frames the wallet's disappearance as something that happened to it, while なくした財布 ('the wallet she lost') emphasises her own action. Both work, but なくなった softens the speaker's responsibility — a quietly common Japanese strategy. 探す ('look for, search') in ~ている marks the ongoing search.