Your daughter must have formed a close relationship with her grandparents and others. It is likely that she is missing them a lot. Give her a chance to talk to them on the phone and to be assured that they are there and missing her too.
Perhaps on the first day after the holiday, she may have had a scrap with another child over a toy or been pushed around by a stronger child in the school. A two year old may not be developmentally ready for a great deal of interaction with children her age. They are often happier to be with adults. In all these behaviours, there is a wide variation and all children are not alike.
Try and spend more time with her to reassure her that the world and the people around can still be trusted. If her problem persists, you may wish to contact her Paediatrician and ask his opinion. She will become herself, but gradually.