If decomposers are absent from an ecosystem, dead organic matter (dead plants and animals) will accumulate and will not be broken down. As a result, nutrients will not be returned to the soil, making it infertile, and the natural recycling of materials will stop completely.
Examiners expect you to mention two linked points: (1) accumulation of dead organic matter, and (2) nutrients not being recycled/soil becoming infertile. Just stating "dead matter will pile up" alone may not fetch full marks. The role of decomposers is to break down complex organic matter into simple inorganic nutrients that are returned to the soil for reuse by producers — absence of decomposers breaks this cycle.