She must repent to Allah, may He be Exalted, from her negligence and she must make up these days, working out the number of days that she is sure will cover all the missed fasts even if it is a little more, to be on the safe side, and she should not underestimate it, because what she did was clearly negligence regarding an act of worship that is one of the pillars of Islam.

Based on that, if you got up before the adhan of Fajr and formed the intention to fast, your fast is valid. If you got up after the adhan, then your fast is not valid as a make-up fast, because your intention was conditional and was not firm. For more information, please see the long answer.

Making up missed fast in order is more prudent, so you should make up what you missed of Ramadan 1439, then what you missed of Ramadan 1440, then what you missed of Ramadan 1441. But for one who does not know the order, or has forgotten it, and has made up what he owed of fasts from previous years, there is no blame on him and his making up of the fasts is valid.