why has Sonia stopped laying?

My 2 year old sultan has been laying every day for the last month or so and now she's just stopped, I haven't had any eggs from her for about a week. Her friend Dorothy was pts a few weeks ago (think she had mareks) but Sonya still has two other hen friends and Ken the cockerell. She seems quite healthy, eating, drinking, playing, everything normal except the egg laying. Anyone any suggestions as to what is going on? I hope it's not the start of another mareks victim