Wrongfully charged R22000 for Savings Account

I have a savings account with FNB that i hardly utilised as it was my secondary account. That account went to -R22000 on balance, which is impossible as i would deposit funds to utilise it. It was not linked to any other products , nor were the service fees that much. Now they handed me over to collectors who are harassing me. How can a savings account thats rarely used be overdrawn by R22000