Zakat is a binding right owed to Allah and to the poor, and it does not lapse simply because years have passed; you must repent and pay what you missed. Go back over each year as best you can, estimate your zakatable wealth at each year's due date, and pay 2.5% for every one of those years, erring on the higher side where records are gone, since the unpaid zakat remains a debt you owe. There is no extra expiation or fine beyond paying it and making sincere tawbah, and if you cannot pay it all at once you pay it off progressively as you are able. Allah warned severely against withholding zakat: "And those who hoard gold and silver and spend it not in the way of Allah — give them tidings of a painful punishment" (Quran 9:34) — which the Companions explained as wealth whose zakat is not paid — and the Prophet ﷺ described the wealth of a man who withheld its zakat being turned on the Day of Judgement into a bald-headed serpent that seizes him.
Q&A · Zakat
I didn't pay zakat for several past years — what do I do now?
Evidence — the daleel
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.