Praise be to Allah.
If a person vows to fast for seven consecutive days, he must fulfil the vow and fast these days consecutively, and it is not permissible for him to fast them separately, because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever vows to obey Allaah, let him obey Him.” This was a vow to obey or worship Allaah. What the questioner says about having started to fast these days but she was unable to fast them consecutively, means that she failed to fulfil her vow. If she was prevented from fasting them consecutively by some excuse, as she said, then she has the choice between two things: either to start over and fast the days consecutively, or to complete the remaining days, but she will have to offer expiation for breaking an oath (kafaarat yameen). The expiation for breaking an oath is well known; it is freeing a slave, or feeding ten poor persons, giving each poor person half a saa’ of the local staple food, or clothing ten poor persons, giving each one clothing that will cover him when praying. She may choose one of these three things. If she cannot do any of them, then she must fast for three days.