There are a few possible reasons for space colonists to lose their technology. One reason could be a natural disaster that destroys all their technology. Another reason could be that the space colonists are unable to maintain their technology and it gradually breaks down over time.
A third reason could be that the space colonists are cut off from the rest of the world and are unable to trade for or otherwise acquire new technology. This could happen if they are stranded on a planet that is too far away from other colonies or if they come into conflict with another group and are cut off from communication.
Whatever the reason, losing their technology would be a major setback for space colonists. They would have to start again from scratch, which would be very difficult.