"Went downhill" is an idiomatic expression that is widely used to describe a decline or deterioration in a situation, person or thing. However, there are several other synonyms or similar phrases that can be used to convey the same meaning. Some of them include: took a turn for the worse, hit rock bottom, plunged, deteriorated rapidly, declined sharply, went to pot, went to the dogs, degenerated, experienced a downfall or slipped. Each of these expressions emphasizes the rapid or sudden decline of something that was once good or desirable. By using these synonyms, you can add variety and depth to your writing, whether it be in academic, professional or everyday situations.