Tuna Acciaioli

Hello - today I received a £1,300 fine for late filing of Self Assessment return, even though my only source of income is my salary. HMRC never sent me any letters prior to this. The gov.uk website says I only need to file SA if I am self employed or have other income. I am in a real panic as I cannot afford this fine and don’t know what to do. Please can you help?