Can a lawsuit be filed against a person and a judgement entered without the defendant's knowledge? We received no summons or any court documents. If not, where do we go from here?

That is a yes and know, a person is suppose to be served with a court summoms, but if for some reason they did not have a address for you, or if they lied to the court and said they did not have an address.
But you are suppose to be served.

Hello DD:

If you look around here, you'll find some great advice from mr yet about what you should do.

excon

Was a judgement entered against you? If a notice is generated, whether you receive proper service notwithstanding, and you don't appear in court, then a default judgement is entered against you. If this is what happened then you need to file a motion to vacate judgement due to improper service.