Account Frozen - bank not helpful to resolve issue

My business account is frozen for 9 days now without any reason or warning. After numerous calls to customer service, ***** department and collection department, while being referred from one to another department, they still couldn't give me a reason. They said I should email the ***** department for a reason. The only reason they gave me is that my account was put on hold after receiving a complaint from someone until I can send them proof of investment to the bank. But they still didn't tell who complaint and what type of proof do they want. Now my business is standing still with all funds frozen, losing deals. And no help from FNB to resolve the problem.