I received a judgement from a credit card debt that was 6 years old. I never was subpoened. What do I do.

It will be valid until you do something.
You go down to the courthouse and see how it was served, there are various levels of service. Did they know where you lived, if not service by publicication could be allowed.
Was anyone served in a residence of yours, subsitute service.

So go down and see how they clamined you were serviced,

Then you ask for a hearing for improper service.

You can try to get the judgement vacated on grounds of improper service. The information you received should include the court and docket number, so you can take that to the court to find out how you were served. However, is your memory that good that you know you weren't served 6 years ago?

But if you find service was improper then, by all means file a motion to vacate. The judge will hold a hearing and, most likely, your motion will be granted.

And then the plaintiff will have a new subpeona and will serve you before you leave the courthouse.

So, all you really accomplish is a delaying tactic and the opportunity for a hearing. Now, if you think you can disprove the plaintiff's case then this could be an advantage to you. Otherwise the plaintiff will just get a new judgement. So you might want to just try and negotiate with them.