What do you do if you do not answer a civil summons?

The precise answer generally depends upon the amount in controversy and the rules of court. For example, some states have a Pre-Trial Conference for small claims cases (such as those involving less than $2500--but check with the Clerk of Court for the dollar limit).

In those kinds of cases, frequently you have to appear at a pre-trial conference rather than immediately file an answer. You will have to file an answer within the time allotted if the judge requires it. If you do not, you will be deemed to have admitted the allegations of the Plaintiff's claim and a judgment may be entered against you.

In larger dollar-amount cases, and in those which seek "equitable relief" (such as a divorce), a written answer must be filed. Again, the failure to do so is tantamount to an admission of the material allegations of the complaint.

Depending upon the nature of the case, a default judgment for the amount sought by the Plaintiff will result from not answering. In other cases, the Plaintiff will be required to go an extra step to prove actual damages, such as in a case seeking damages for bodily injuries. You, as the defendant, have to be notified of a hearing on that second aspect of the case and be given a chance to defend.