How does one make up for prayers missed due to negligence for a long, unknown period? And should secret prayers be made up secretly and audible prayers audibly, or is repentance and a fresh start sufficient?

Prayer is the pillar of the religion, and abandoning it is a grave sin that necessitates immediate repentance to Allah Almighty, future adherence to it, and abundant seeking of forgiveness and righteous deeds. The one who abandons prayer must immediately make up the missed prayers, whether they were intentionally or unintentionally omitted, and whether they were to be recited aloud or silently. If the number of missed prayers is unknown, then one must continue to make them up until their conscience is clear and doubt is removed.