Fnb I'm not happy with your services

I have an FNB cheque account and on Sunday 4th April 2021 I transferred R500 from my savings to my cheque account. My balance was R40 it then became R540. I then swiped R140 and my balance was R400. A day later on the 6th of April my balance is R100 and R300 has disappeared into thin air. There is no deduction, there's no debit order there is not bank charges as I pay those monthly in one transaction. FNB I want my R300 back please its a lot of money for me I can buy groceries for my kids for R300. I had no pending payments or debit orders. I keep track of all my banking transactions nothing was outstanding so this 300 just disappeared from my balance and there is no Trace of it. Please refund me asap.