Bankruptcy Law Questions? Ask a Bankruptcy Lawyer.
Is there a default provision in the promissory note? If there is you could exercise that clause as soon as possible. Once they default you can sue them for the note, get a judgment and execute on any bank accounts, property or wages. However once you did this it would most likely prompt them to file bankruptcy that much sooner.
If they do file bankruptcy, you will be an unsecured creditor, and will need to file a claim just like every other creditor to whom they owe money. You may get something if there is anything to distribute, or if they must file a Chapter 13 plan you may get a small portion of what is owed. Definitely file a claim if they do file, but that is about all you can do. If they go bankrupt your note will be discharged. If there is a problem with their bankruptcy and they don't get discharged however, then you should proceed with collections.