Collection on prescribed account

Good day

The Collections department deducted R5000!! from my personal savings account. Upon enquiry it was for an old account dating back more than 5 years. I requested a statement, proof of acknowledgement/issue of summons. To date I received the statement wherein the last traceable paid amount dates back to 2013, there are a few questionable payments after that but no one can tell me its origin as it wasn't done by me or anyone instructed on my behalf. I have requested a reversal but denied, is this lawful as per prescribed accounts? I would like my R5000 back please.