A blocked sink can be caused by various factors, and it's essential to identify the root cause of the blockage to ensure that it doesn't happen again. Here are some of the most common causes of a blocked sink:

1. Food Waste - When food waste is disposed of down the sink, it can accumulate in the pipe and cause a blockage over time.

2. Soap Scum - Soap scum is a common cause of blockages in a sink. When soap scum accumulates in the sink, it can cause the water to drain slowly, leading to a complete blockage.

3. Hair - Hair is a significant cause of blockages in bathroom sinks. If you're shaving or washing your hair over the sink, hair can quickly accumulate in the pipe and cause a blockage.

4. Foreign Objects - Foreign objects, such as baby wipes or sanitary towels, can also contribute to a blockage in the sink.

5. Grease - Grease is one of the most stubborn substances. It can quickly solidify and cause a blockage.

Regardless of the cause of the blockage, it's crucial to address it immediately to ensure that it doesn't cause a complete blockage that will require serious remediation in the future.