Worries in big 4.

Now I am working for Big 4. I got my offer last December. However, I have been unassigned for several weeks. I do not know the reason why? Foreigner or something? Actually I do not know to whom I should seek for advice. I know there is high turnover in these companies and just want to ask whether long-time unassignment will lead to automatic dismissal? Even though I do not want to stay there for a long time, I should at least stay there for a year. What should I do? Thanks for your help.