If your child is four or older, then he or she should be able to spend some time in a preschool environment with other kids with no real problems. Assuming that the school is an enjoyable place with block play, sharing times, art experiences, and so forth. If it's more highly structured, with academic expectations, this could be at the root of the problem, and your child may need a school environment more closely tailored to their individual developmental needs. However, even then, some kids have more trouble making the transition from home to preschool environments, and for these kids, some sensitivity is called for.
First, consider scaling back the involvement in preschool. If a full day now, then a half day; if a half-day now, then a quarter of a day.
Second, help make the transition by spending some time with your child at the school to start with (perhaps you could volunteer a half hour at the start of each day). Then gradually phase out your involvement as they become more comfortable. Or, get to school early, and spend some time with your child in the classroom environment, helping them to feel comfortable in it. If they get upset, let them know you'll be outside the school when class begins, and if they need to talk with you, let them know that you'll be available. Gradually phase this out as they become more comfortable.
If your child continues the same level of crying, or even becomes more upset, then consider postponing for a year or so the involvement in school. This reaction may be telling you that this child is simply not ready for school.