"忽然唱起歌来" can be translated into English as "suddenly started singing" or "burst into song".
"Suddenly started singing" is a straightforward and commonly used expression, directly conveying the unexpected beginning of singing.
"Burst into song" is a more vivid and idiomatic phrase, implying a sudden and enthusiastic start to singing, often with a sense of spontaneity or emotion.
Both translations are appropriate depending on the context and the desired tone of the sentence.