Severed contract and refusing to tell me why - Background Investigator Peraton Employee Review

One week after training (when they were perspecta) my contract was severed. I was told NBIB would let me know why. 5 years later I have yet to hear anything. I've sent a few emails and have gotten the conversation started then they stop replying.. They want me to pay $7000+ for training I attended. Since they will not tell me why I was let go I haven't paid because it seems like a scam. This has since affected my credit score..