C.T.
Hi T. - How unfortunate that a long working relationship has to end this way - I just dont understand how people think that if they avoid a situation, it will just go away. I'm not an attorney but I'll give you what I've learned from experience in these things.
You're first step should be to send them a letter, certified mail, itemizing exactly what they owe you and the date you expect payment by before pursuing legal action.
You might need to call and interview a couple of attorneys before you find a good fit. You might also consider one of those "prepaid legal" plans to keep the cost down. Otherwise, you will pay him hourly because mostly they will be helping you with the notifications and filings. This is very unlikely it will go to a jury or anything. The amount of the claim determines where your case should be filed - County Court/small claims or in District Court. I think in Colorado the amount is $10k.
As far as a filing a lien on their property - this can only be done if the work completed was attached or related to their property like architectural design, mechanical repair, construction, landscaping etc.