。
She sang a song whose title I didn't know.
Literal
She [topic-は] I [subject-が] title [object-を] know-not song [object-を] sang.
A neat example of the gap-style relative clause that Japanese uses pervasively. The clause 私が題名を知らない ('I don't know the title') sits before 歌 — its 'gap' position (the song itself) is implicitly the head noun. English needs 'whose,' 'that,' or 'which' to fill that gap; Japanese just puts the modifier in front and lets the listener supply the relationship. Inside the modifier, が marks 私 as the embedded subject. 題名 (だいめい, 'title') is the formal Sino-Japanese reading; the more casual everyday word for a title is タイトル.