This question was put to Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him) and he said:
According to the most sound opinion, if a person is unconscious and is in a coma because of sickness or otherwise, he is not obliged to pray, so he does not have to make up the prayers he missed. As for fasting, he does have to make up the days that he did not fast whilst he was in a coma.
The difference between prayer and fasting is that prayer is done repeatedly, so if a person does not make up the prayers he missed, he will pray on the following day. But in the case of fasting, it is not done repeatedly. Hence a woman who menstruates has to make up the fasts she misses but not the prayers.