Children sometimes experience something called selective mutive disorder in new and sressful situations. These children literally do not have the ability to speak-the words will not come to them. Be patient! We are always giving children mixed messeges about how to handle certain situatuions..."Use your words- don't hit"...VS. "Just be quiet and respect the adults around you." Kids need to feel safe in order to assert themselves and speak up. It would only6 take one time for a child to be reprimanded or not believed for them to shut down and not want to respond- out of fear of reprimand or disbelief. Try talking to your daughter in a "safer" non threatening environment" Is she like this at home? Or, only in "outside" situations. Give her opportunities to speak her mind and feel successful and not intimidated at home. Than move on to the "outside" world and start small.
Good luck

i'm in a similar situation, my daughter is 4 and attending the local nusery which is attached to the primary school. she is shy and if she is told not to do something she usually shuts down and her teachers cannot communicate with her. talking to her during these stressful situations doesnt work. i feel she needs time and patience, but they suggested a special needs counselor comes in and observes her. i'm pretty upset about it.