whathappens if i ignore a complaint? or a lawsuit?

If the person ignoring the lawsuit was properly served, and is subject to the jurisdiction of the court (i.e. lives in the jurisdiction or has other significant contacts with it), a default judgment will be entered against him based solely on the evidence presented by the plaintiff.  Indeed, in many cases, the plaintiff will not even have to present evidence, other than his own word. That judgment can be enforced (e.g. through levying on assets, garnishing wages, etc.) exactly as if the case had been fully litigated.  At that point, the defaulting person could seek to have the judgment vacated, but that will be quite difficult, and even if he succeeded, he would merely be back at the beginning, fighting a lawsuit, having gained nothing.  Also, before it will vacate the default judgment, the Court might impose conditions, such as requiring the defaulting party to deposit money into court to cover the damages if the person loses the lawsuit.  All in all, it is a very bad idea to ignore a lawsuit against you.