A Startup Stole My Code After 300 Hours Of Work. What Are My Options?

Throwaway account - I want to be vague until I better understand my options

I was working for a startup and after 300 hours of software development work over 2 months we couldn't agree on an equity negotiation - after stating a contracting price for my work (15k) they extremely low-balled me ($2k) and when I refused they locked me out of everything and are still using my code. There was no contract or anything signed, just a word of mouth agreement initially (Yes, I am naive).

Their company is a travel startup that relies on a "secret sauce" which is a loophole on airline websites. They are worried about a competitor reverse-engineering their strategy.

Their one mistake is that they never got around to me signing an NDA. Would it be advised for me to get a lawyer and try to reattain my IP? Is it possible to pay a lawyer to contact all major airlines and make them aware of the loophole and have it covered in TOC so if the business succeeds, the airlines can sue them? What other options may I have?