Hi there,
To better understand the problem, more insight and details are needed into circumstances in which she asked for separation and more on how her behaviour, speech, activity level has changed over the days or months and above all when did u start noticing changes in her. Details on her sleep pattern, and any recent changes in her social relationships viz. at workplace, with neighbours, other family members.
Next Steps
You need to get along with ur in-laws and visit a Psychiatrist with all description of ur wife's condition.
Health Tips
You or someone she trusts needs to talk to her and get to know what she thinking, believing.
If she's abnormal psychologically, better avoid arguing against her beliefs or forcing her in any way, as it would only make her distrust u further.