What should be done when a husband withholds his wife’s rights in bed?
Question

                I find many topics concerning moslem women and the punishment of ignoring or avoiding their husband in sexual matters.But I would like to know the rights of a woman when this problem is visa-versa, and it's the husband who is denying the wife of her right. And if the husband is ill, does he still not have the obligation towards his wife?
Praise be to Allah.When a husband deprives his wife of the right 
  to intercourse, the scholars call this eelaa’. Eelaa’ means 
  the vow of the husband who is able to have intercourse never to have intercourse 
  with his wife, or not to have intercourse with her for more than four months. 
  The evidence for that is the aayah (interpretation of the meaning): 
“Those who take an oath not to have sexual relation 
  with their wives must wait for four months, then if they return (change their 
  idea in this period), verily, Allaah is Oft-Forgiving, Most Merciful[al-Baqarah 
  2:226]  
It was narrated that Ibn ‘Umar (may Allaah be pleased 
  with him) used to say concerning eelaa’: “If the period of eelaa’ 
  expires, then the husband has either to retain his wife in a handsome manner 
  or to divorce her as Allaah has ordered” (al-Bukhaari, al-Talaaq, 
  4881)  
Eelaa’ is forbidden in Islam because it is a vow not 
  to do something that is obligatory. It is in effect a vow not to have intercourse 
  with one's wife, either never again or for a period that exceeds four months; 
  or else it means vowing that if one’s wife does not do a certain obligatory 
  action or does a certain haraam  action, he will not have intercourse with 
  her. The fuqahaa’ also said that the one who does not have intercourse 
  with his wife for more than four months without swearing an oath, in order to 
  harm her and without having  a valid excuse, also comes under the category 
  of eelaa’.
 The ruling on this matter is: that if 
  he has intercourse with his wife during that period, then he has “returned”, 
  because “returning” in this context means intercourse and in this 
  manner the woman has attained what she is entitled to from him. If he refuses 
  to have intercourse after the stated time limit has expired, then the judge 
  should order him to divorce her if his wife requests that. If he refuses to 
  have intercourse and he refuses to divorce her, then the judge should issue 
  a divorce and annul the marriage. 
 And Allaah knows best.  
For more information refer to Zaad al-Ma’aad 
  by Ibn al-Qayyim, vol. 5/344  
If it is the case that the husband is sick, please see 
  Question No 1859, 5684