She made du’aa’ and her prayer wasn’t answered, and she said, There is no God
Question

                A few years ago I made du'aa to Allah to provide something to me. However, my prayer wasnt answered. I became mad and said that there is no Allah. Now I regret that I said that, because I understand that what I said was shirk, but can I say my shahada again, and repent fully, so I can be a Muslim again? Also, is that even shirk? Because I said that when I was mad, or does that still count?.
Praise be to Allah.What the questioner said is the essence of 
    kufr. She should have controlled herself and watched what she said and did at the time of anger, for it led her into kufr. 
Hence we think that she should say the 
    Shahaadatayn with the intention of entering Islam, if she was in full control of her mind at the moment when she said those bad words. Anger is 
    not an excuse unless she had lost her mind  to such an extent that she did not understand what she was saying. 
The Prophet  (peace and blessings of 
    Allaah be upon him) stated that the Muslim may speak a word that may lead him to Hell and bring upon him the wrath of Allaah. It was narrated from 
    Abu Hurayrah that the Prophet  (peace and blessings of Allaah be upon him) said: “A person may speak a word that pleases Allaah and not pay 
    any attention to it, but Allaah raises him many degrees in status thereby. And a person may speak a word that angers Allaah and not pay any 
    attention to it, and He may throw him into Hell because of it.” 
Narrated by al-Bukhaari, 6113. 
According to the version narrated by Muslim: 
    “A person may speak a word, not realizing (how serious it is) and be thrown as far into Hell because of it as the east is from the west.” 
If you enter Islam again by uttering the 
    Shahaadatayn, and if you regret what you said, then there is the hope that the good deeds you have done will not be lost. 
It was narrated that Hakeem ibn Hizaam (may 
    Allaah be pleased with him) said: 
I said, “O Messenger of Allaah, there are 
    some things that I used to do as acts of worship during the Jaahiliyyah, such as giving charity, freeing slaves and upholding the ties of kinship. 
    Will I get any reward for that?” The Prophet (peace and blessings of Allaah be upon him) said: “You entered Islam with the good that you had done 
    before.” Narrated by al-Bukhaari, 1436; Muslim, 123.  
So you and all our Muslim brothers and 
    sisters have to take the matter of religion and belief seriously. Because a person’s religion and the soundness of his belief are his capital by 
    means of which he attains happiness in this world and in the Hereafter, and the good pleasure of the Lord, may He be glorified. 
Secondly: 
Whoever makes du’aa’ to his Lord receives an 
    answer in all cases. The answer to prayers is not only getting what one asked for; rather there are two other responses that may come which are: 
    warding off evil from the person who made du’aa’, commensurate with his du’aa’, and storing up the reward for the du’aa’ which the person will 
    receive on the Day of Resurrection.  
Allaah has promised us, on the lips of His 
    Messenger  (peace and blessings of Allaah be upon him), that whoever calls upon Him, fulfilling the conditions of du’aa’ will receive one of 
    these responses. It was narrated that Abu Sa’eed al-Khudri (may Allaah be pleased with him) said: The Messenger of Allaah  (peace and 
    blessings of Allaah be upon him) said: “There is no Muslim who calls upon Allaah with a du’aa’ in which there is no sin or severing of family 
    ties, but He will give him one of three things: either He will answer his prayer, or he will store up an equal amount of good (reward) for him, or 
    He will ward off an equal amount of evil from him.” They said, “O Messenger of Allaah, then we shall say a lot of du’aa’?” He said, “Allaah most 
    Generous.” 
Narrated by Ahmad, 10709. al-Mundhiri classed 
    its isnaad as jayyid in al-Targheeb wa’l-Tarheeb, 2/479; al-Haafiz Ibn Hajar classed it as saheeh in al-Fath, 11/115. 
All of this is what is meant by the verses 
    (interpretation of the meaning): 
“And your Lord said: ‘Invoke Me [i.e. 
    believe in My Oneness (Islamic Monotheism) and ask Me for anything] I will respond to your (invocation). Verily, those who scorn My worship [i.e. 
    do not invoke Me, and do not believe in My Oneness, (Islamic Monotheism)] they will surely enter Hell in humiliation!’”
[Ghaafir 40:60] 
“And when My slaves ask you (O Muhammad) 
    concerning Me, then (answer them), I am indeed near (to them by My Knowledge). I respond to the invocations of the supplicant when he calls on Me 
    (without any mediator or intercessor). So let them obey Me and believe in Me, so that they may be led aright”
[al-Baqarah 2:186]
It may be better for the person not to get 
    what he asked for his du’aa’, because of evil or fitnah that may result from his getting it, of which he is unaware. So Allaah wards that off from 
    him and gives him something that is better for him in this world, by warding off evil from him, or something that is better for him in the 
    Hereafter, by storing up the reward of the du’aa’ for him. 
Shaykh al-Islam Ibn Taymiyah said: 
The du’aa’ in which there is no transgression will be answered, or else the person will be 
    given something equal to it. This is the best response. For the thing that he asked for may be unattainable or may be harmful to the person who 
    prayed or to someone else, but he is ignorant and unaware of the harmful elements in it. But the Lord is Close and Ever-Responsive, and He is more 
    compassionate towards His slaves than a mother towards her child. And He is the Most Generous, Most Merciful: if He is asked for a specific thing 
    and He knows that giving it is not in the best interests of His slave, He will give him something of equal value, as a father does for his child 
    when he asks him for something that is not good for him, so he gives him something of equal worth, and for Allaah is the highest description. 
Majmoo’ al-Fataawa, 
    14/368. 
And Allaah knows best.