When a person is in the state of major ritual impurity, he is obliged to perform ghusul! If there is no water, he must perform tayammum! Once he finds water,he must perform Ghusul! Therefore, if this person forgot and didn’t perform ghusul when he found water, his prayers are invalid and he must perform them again!

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Depends on what extend. However you will not be rewarded for that prayer except the part you were concentrating in.

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