In general, if you do not answer a civil complaint, you are likely to lose the case by default. That is, the judge will rule against you without hearing your side of the case in any way whatsoever. When a complaint is made against you, you typically have a deadline by which you must reply to that complaint. If you do not reply, the person who made the complaint against you can apply to the court to be granted the judgment by default. If the judge grants this the person who filed the complaint will get everything that they have asked for in the complaint. In other words, you will be liable for whatever damages they have asked for. Therefore, it is vital that a person should answer any civil complaint made against them.
If a party fails to answer a complaint then the plaintiff can obtain a default judgment. But that does not necessarily mean the case is over, particularly where damages are estimated or unspecified. This then would only prevent a defendant from challenging the facts as alleged in the complaint. Let's say in a car accident case the defendant concedes liability and allows the judgment. There would still be a trial on damages. The Defendant just couldn't deny liability.
Also a defendant can always attempt to open or set aside a judgment. Let's say the Plaintiff violated a provision of the Soldiers and Sailors Relief Act, knowing the Defendant was in Iraq.