What should one who, out of ignorance, issued a fatwa that a veiled woman receives no reward, and then regretted it, do?

You must hasten to sincere repentance by regretting what you have done and resolving not to speak about Allah's religion without knowledge again, because speaking about Allah without knowledge is one of the gravest sins. Allah says: "Say, 'My Lord has only forbidden immoralities – what is apparent of them and what is concealed – and sin, and oppression without right, and that you associate with Allah that for which He has not sent down authority, and that you say about Allah what you do not know.'" (Al-A'raf:33). Sincere repentance erases what came before it. Allah says: "Except for those who repent, believe and do righteous deeds. For them Allah will change their evil deeds into good ones. And Allah is Oft-Forgiving, Most Merciful." (Al-Furqan:70).