Interest charged to my 6 month interest free account

I have had a 6 month interest free account with Edgars for the past 30 years. I consider myself a good loyal client and I always keep my account up to date. This month for the first time ever I see that Edgars charged me interest on my account. I wrote to the Edgars accounts department and have received no response yet. This is unacceptable and I want this sorted out ASAP please