The plaintiff can file a motion for default. You will be served a copy of this motion. If granted, a Motion for default means you lose the case. Procedurally an attorney can then request that the court "vacate" or in effect re-open the case so you can answer and the case can go forward. It may or may not be granted. That means you can lose the case by doing nothing. The court can order you to pay damages to the plaintiff. That's kind of a lot of legal answer, which may or may not make sense to you.
Here's the practical answer: You can't ignore a lawsuit. If 20 days has already gone by get yourself a lawyer immediately and file an answer. Have the answer filed both with the court and served on the plaintiff by a proper process server as soon as possible. If you own a business and this is a matter that happened at work talk to your insurance company and see if you have coverage. If so, they will appoint an attorney for you. If this is a matter that happened at your job you should talk to your supervisor immediately. If this is a private matter you need to hire your own attorney. These are not typically cases that are done by legal clinics or for free.
You should never ignore a lawsuit. If a default judgement is ordered against you, you may have to pay damages and costs without ever having presented your case or defending yourself. Once this happens you are pretty much out of luck.
Another word of caution. Do not UNDER ANY CIRCUMSTANCES contact the plaintiff in this case. Don't call, email, write a letter, text, do anything. If the plaintiff is represented by counsel you are permitted to contact that person but my strong advice is to hire an attorney and let that person deal with the attorney directly. Given the nature of this kind of case you do not want to have any communication or the appearance of communication with the plaintiff. Don't have any of your friends, relatives or co-workers make contact either.
This email response does not constitute an attorney client relationship. The comments here are purely advisory and do not represent a complete legal analysis of the problem presented above. The person requesting information is strongly advised to hire an attorney in his or her community.
Actually, in Washington, if you don't answer, the Plaintiff does not need to serve you with a motion for default. In most cases a judgment can be entered "ex parte" (without you being there). So you should definitely file an answer within the 20 days or you will most likely lose the case and have a difficult if not impossible time overturning that. "File an answer" in WA means serve an answer to the Complaint (answering each paragraph) and sending it to opposing counsel and filing it at court. I suggest hiring an attorney immediately if you have not already done so, as damages requested by Plaintiff may be extraordinary.