You have to first obtain the medical documents pertaining to her mental ill health, secure them.
You can obtain medical examiner's opinion related to her health and the possibility for getting cured if proper treatment given and she follows the medical recourse properly.
You should issue a legal notice stating that she deserted on her own from the said date and is either not willing to return to the matrimonial fold or is interested in leading the married life with you.
You can also state that you are still ready and willing to take her back and are ready to give her proper medical treatment to her mental health problems she is suffering now provided she returns to the matrimonial home within a time stipulated in the notice. You can also mention the plight of the innocent infant which suffers the loss of maternal love and affection due to her decision to desert both the child and her matrimonial home and discontinuing the married life with out any valid reason.
A legal notice clearly containing all the details should be sent to her address by registered post.
Secure all the evidences for future litigation or even a divorce to be contemplated in this regard.
You may decide after giving one more chance to save your marriage to continue with your married life or to dissolve the marriage by a decree of divorce on the grounds of mental cruelty and desertion.
Consult your parents and a local lawyer on all such further steps in this regard and decide patiently especially in view of the infant child's interests and its future