If I never received UD complaint, only a sheriffs notice to vacate, what can I do?

On Friday, November 1st, I came home to a sheriffs notice to vacate by Nov. 7.  I have not been served anything prior to this. I looked up the UD case number and it says I was served through substitute mail service but I can not and did not  receive any mail at my address. USPS does not deliver to my house so I receive mail through my PO Box. After further research, it says that I should have had 15 days to respond to the complaint if it was sent via US mail. However, my landlord filed for a default judgment after only 14 days and the clerk awarded this the same day. I then noticed another UD case against me about 4 weeks prior to this case and it is pending dismissal because it says received returned mail for my address as the post office told them they are unable to forward to my address. For some reason, the most recent case does not show it was returned, and it shows that I never filed an answer, so they were awarded a default judgement by the clerk. So I have the sheriff coming to force me out on Thursday at 6 AM. However, there are two existing UD cases against me. I was never served anything at all. And even if I was served, they did not allow me my full 15 days to file an answer. It shows they mailed the complain on October 15 and granted default judgement on October 29. What can I do to stop the sheriff from evicting me on Thursday? What can I do to defend myself against the UD case, being that I was never served? I am not sure what's the best option to  take. I want to prevent the sheriff from forcing me out so I have time to properly defend my rights now that I am aware there's is an UD lawsuit against me. Do I need to file an emergency motion to STAY the default judgement with my answer included? Then should I file a separate motion to vacate the default judgment due to lack of notice? Or should I include my answer here?Should I file a separate motion to point out the clerks error in not letting me respond in time?