Always have a contract, even for smaller bits of work. That’s the foundation of a good business relationship.

If they have the money to pay, but are ignoring you then escalate from your contact at the company to someone higher in the chain. If that doesn’t work and it’s a relatively small amount of money then you can try to enforce the contact.

I’m in the UK and there’s a small claims court for specifically this. I’ve not gone through it myself but from those that have it’s simple enough to do yourself without paying a lawyer, but it’s time consuming. Im not sure where you are but look into if there’s in in your jurisdiction.

If the client likely doesn’t have the money to pay, then it’s likely a waste of time to pursue it. I had this happen recently when a client went under and ended up just writing it off as a loss. Luckily it was only my time that was wasted so not the end of the world.