I was sued for 1900 dollars on an old card, got a judgement against me, and agreed to pay the money by the 15th of August. I may not be able to do that now due to unforeseeable circumstances. What happens if I'm unable to pay by the deadline?

They have the right to enforce the judgment by any legal means - garnishment, seizure of bank accounts.

You have twice violated their "trust" - once by not paying the debt and once by not honoring the subsequent agreement - and I would think they are out of patience.

They may - if you notify them and make another arrangement - be willing to work with you but I wouldn't count on it.

They can use the judgement to attach your assets or garnish your wages. I would contact them immediately and tell them you are havingt a problem getting the whole amount, but that you can pay $X by 8/15 and the balance by a later date.