Expiation (Kaffara) only becomes necessary upon an individual if a fast was broken deliberately after actually starting it by eating, drinking or having sexual intercourse. As such, if a fast of Ramadhan was not kept altogether, then although one will be sinful for not fasting, a Kaffara will not be necessary, rather one will be obliged to make up for the missed fast (qadha).
The great Hanafi jurist (faqih), Imam al-Haskafi (Allah have mercy on him) states while discussing the acts that make one only liable to make up for the fast (qadha) and not expiation (kaffara):
“If an individual broke his fast by mistake, such as whilst gargling water entered into his mouth unintentionally…or…..or he woke up in the morning without making an intention of fasting….then in all these situations, only a Qadha will be necessary.
Allama Ibn Abidin (Allah have mercy on him) explaining the above states:
“(Or he woke up in the morning without making an intention to fast)…..Because a Kaffara is only necessary upon a person who broke the fast after keeping it…. (Only a Qadha will be necessary) meaning there will be no Kaffara. (See: Radd al-Muhtar ala al-Durr al-Mukhtar, 2/401-406)
Therefore, in light of the above, the one who missed the obligatory fasts of Ramadhan should firstly repent to Allah Almighty and seek his forgiveness for not fasting. Secondly, it will be necessary to make up (qadha) for the 400 fasts that were missed, although a Kaffara will not be necessary. The person concerned should begin making up for the missed fasts as soon as possible, Insha Allah.
And Allah knows best