She prayed and forgot to remove her nail polish

In order to wudu to be valid, it is essential to remove anything that prevents the water from reaching the skin, such as oil, nail polish, wax or band-aids. Prayers offered in that state are invalid and not acceptable. 

Al-Nawawi (may Allah have mercy on him) said in al-Majmoo‘ (1/493): If there is wax, dough, henna and the like on one of his limbs, which prevents water from reaching any part of it, then his tahaarah is not valid, whether the amount is large or small. If there are traces of henna and its colour left on the hand, without there being any solid material left, or elsewhere, or traces of liquid grease whereby water flows over the limb but does not stay there, his tahaarah is valid. End quote. 

It says in Fatawa al-Lajnah al-Daa’imah (5/218): 

If the colour is impermeable, then wudu is not valid unless it is removed before doing wudu. If it is not impermeable, then wudu is valid, as is the case with henna. End quote. 

Based on this, if a woman prays and there was anything on her that prevented water reaching the skin, and she forgot to remove it before praying, then she is not regarded as sinning because she forgot; but the prayer is not valid and she has to repeat wudu and the prayer, because of the report narrated by Muslim (359) from ‘Umar ibn al-Khattaab (may Allah be pleased with him) who said: A man did wudu and omitted a place the size of a fingernail on his foot. The Prophet (blessings and peace of Allah upon him) saw it and said: “Go back and do wudu properly.” So he went back then he prayed.” 

Al-Nawawi (may Allah have mercy on him) said, commenting on this hadeeth: This hadeeth demonstrates that if a person omits a small spot of that which must be purified, his purification is not valid, and this is something on which the scholars are agreed. … This hadeeth also indicates that if a person omits washing any part of his body and purifying himself out of ignorance, his purification is not valid. End quote from Saheeh Muslim. 

The Standing Committee was asked: 

If I forget, when doing wudu, to wash a small part of that which is washed during wudu, then I remember it straight after doing wudu, should I repeat wudu or is it sufficient just to wash that part? 

They answered: 

Correct sequence is one of the conditions of wudu being valid. If a person forgets to wash one of the parts of the body that are washed during wudu, or a part thereof, even if it is small, if that happens during wudu or immediately after, and the traces of the water are still on his body and have not yet dried, then he should only wash what he forgot and whatever comes after that. 

But if he remembers that he forgot to wash one of the parts that should be washed in wudu, or a part thereof, after his body has become dry, or he remembers that whilst praying or after he has prayed, then he should do a new wudu from the beginning, as Allah has prescribed, and he should repeat the prayer in full, because there was no continuity in this case and a long time has passed since he finished doing wudu. Allah has enjoined washing all the parts of the body required in wudu, so if he omits any part, even if it is small, of the parts that are to be washed in wudu, it is as if he did not wash it at all. This is indicated in the report narrated by ‘Umar ibn al-Khattaab (ra): The Messenger of Allah (blessings and peace of Allah be upon him) saw a man doing wudu and he omitted a space the size of a fingernail on his foot, so he told him to repeat the wudu and the prayer; so he did wudu again and then prayed. A similar report was narrated by Imam Ahmad and Abu Dawood. 

And Allah is the source of strength. May Allah sent blessings and peace upon our Prophet Muhammad and his family and Companions. End quote. 

‘Abd al-‘Azeez ibn ‘Abd-Allah ibn Baz, ‘Abd al-‘Azeez Aal al-Shaykh, Bakr Abu Zayd  

Fatawa al-Lajnah al-Daa’imah: al-Majmoo‘ah al-Thaaniyyah, 6/92 

For more information please see the answer to question no. 103738 

And Allah knows best.