Get all your records together and a time-line as best you can remember. Get anyone else's stories and any pictures you can find, then call a laywer who does this work for a living.
A contractor who abandons a job usually has no recourse.
Looks like they may be trying to scare you.
If he was a good contractor he would not have just found your file and realized you owed money. How does that happen if you are on top of your projects? Where was the communication?