It's worth noting that if you do resort to pushing the company into bankruptcy, at best you'll get nothing, and at worst you'll make things even more difficult for the founders than they already are.
That is my fear in pushing too hard. I actually do want the company to succeed, but I just don't have the wherewithal to give the company this free loan.
Except, you're very likely not going to get the money, so I hope the "don't have the wherewithal to give the company this free loan" is an exaggeration. You should cut your losses. This is a sunk cost for you. Use your creativity to work on furthering your ambitions and career from here. Live for the future, not the past.
Make it their fear - they are still in business, so somebody is getting paid.
Call them up and tell them straight out that they have a week to get the money otherwise you go after them in court, which will throw the company into banckruptcy.