I just got summons to go to court for an unpaid credit card what is the worst that can happen? Do I have to show up? And what if I don't respond?

Well I guess the worst thing that could happen is two mafia guys come with guns and take everything you own and threaten to kill you if you don't pay more.

Now since most likely that won't happen, you don't have that much to fear. Yes you should always go,

If you don't go they win automatically by default, they don't even have to present their case, if you go, you can make them prove it is a valid debt, you can make them prove it is within the SOL time frame. And you can show ( hopefullY) where you tried to work out payment plans and they refused, and maybe the judge will merely order a payment plan, not give them a judgement. (not often but always worth a try)

But if they get a judgement, depending on what STATE you live in, they may be able to take all the money in all of your bank accounts, leaving any outstanding check to bounce. Next they can get a garnishment of your wages nromally around 25 percent of your pay.