My 4 yr old son cried in preschool too and we couldn't figure out why since he did so well the first few months. Then he kept saying that he missed me. I found out from the teachers that he cries throughout the day too and I realized that he did this before lunch time which is before naps (which he hates!) He also would see kids be picked up at noon by their moms and he thought that his mom should pick him up at this time too.
We started giving him incentives for having a good day at school and not crying when I dropped him off. We made sure to ask the teachers that he wasn't having a problem with other kids. I promised him a "fun afternoon" once I picked him up. He even carried around my husband's business card with him for a week so that he could call daddy if he needed him. Then when he would have a good day at school, we would make a big deal of it and celebrate. He would be so proud of himself that the first thing he would say when I picked him up is "Mommy, I didn't cry at school today!!! Can I call Daddy and tell him?" I also tried to find out stuff they were doing in school and to get him excited about their fun activities so that he would have something to look forward to.
Good luck! Hopefully it is just a phase and she will start enjoying school again. But don't give up!!! She's got 12 more years of school ahead of her. Teaching her to be enthusiastic about school now will help you in the future.