No authority to debit

I took a 6 months loan of 5000 from FNB in November 2020 the first installment was suppose to be in December of which they did not deduct it , so when there was money in my account on the 16 of January 2021 they took it and I could not get it back as they said it’s not reverseble, so after trying to get it back by sending them messages on the banking app and calling them.. eventually all my efforts did not help , so this morning they deducted twice on my account of which they were suppose to deduct once. Can you please help me with this matter, when i lair this complaint they gave me the ref number ***