How do I collect on a promissory note?

I loaned 5k to a neighbor. I have a promissory note with their company stock pledged as collateral. The terms of the loan expired, now, they don't feel like paying me back. They sent cease and desist letter. I told them no, I'm the original owner of the debt, not 3rd party. The stock was useless when they pledged it. Is that illegal? And how go I collect?