What to do when Contractor is $8k behind in work?

[Taylor Jennings]

We didn't set milestones or goals, we paid them based on the work they said they had completed, and now we can't pay them anymore because they are 2-3 weeks behind...

How should we go about getting the contractor to do $8k in work which was already paid for?

(I'm expecting really sad answers...)

My thought was to set specific goals for this week to get done and hope that hey knock it out.

Use some psychology techniques to get them to take hold themselves to a higher standard for themselves... Not sure how it's going to go.