When you are sued, you fight it by filing an answer, admitting or denying the allegations in the complaint. If you don't file an answer, then the person filing the lawsuit--the Plaintiff--can ask the court to take a default judgment against you. A judgment by default means that whatever accusations are made against you by the Plaintiff can be accepted as true, and any amount of money they can "prove up" to the court will be granted.
You don't want to have a party take a default against you. After the default, interest begins to accumulate on these debts, and if left uncollected for years, the interest can amount to several times more than the original principal owed.
Find out what you can do to fight against debt collectors and others who sue you.