Demand Letter from my previous employer

Problem/Goal: I received a demand letter from my previous employer today via email.

Context: I haven’t been with the company for a year already. And I have sent in my immediate resignation after I fell ill during my time with the company. What I don’t understand is that, they are claiming that I owe them xxxx amount of money that I don’t even know where it stemmed from and why I owe them that amount. I don’t have a copy of my contract anymore and I have sent back all the equipment that they provided me. I don’t plan on running away from what I owe them but I was thinking about what should be the next steps to this. I have acknowledged the email and still waiting if they can explain where did it stemmed from. Would love to hear about suggestions or people with the same issues if ever. Thank you.