Blocked main drains are usually caused by a buildup of debris, grease, tree roots, or collapsed pipes. Common signs include slow drainage, gurgling sounds, and sewage backups. To fix a blocked main drain, professional drain cleaning methods such as hydro jetting or snaking are used to remove obstructions. In severe cases, drain replacement may be necessary. If you suspect a blockage, it’s best to consult an expert to prevent water damage and further plumbing issues.