Back in 1987 I didn't pay one of my credit card and so it stayed in my credit for 7 years. I just received a judgment on my credit for the same credit card. Is this possible? This has already ruined my credit 20 years ago. What should I do?

Chances are very good that the statute of limitations has run out for this debt. It still exists but they can not get a judgement againist you in court. Check your state statute. Could you explain further what you meant by "judgment on my credit " Collection companies buy old debt for pennies on the dollar hoping you will blink and make an offer to pay, that offer of even a penny will restart the statute. If the debt was real and you want to pay that's strictly up to you but you can no longer be forced to pay.

Thank you for the answer. They reported the "Judgment" on my creadit report. Is it possible to ruin my credit after 20 years once again? Isn't this against the law?

That area is a bit beyond my experience but I would contact the Credit reporting agency and challenge that portion of your credit report. The statute has run out so I think the reporting of it also has run out. 7 years is the norm.